Add registerFilter method

[didoo]

Issue #, if available: #207 #199

Description of changes:

• introduced a registerFilter method for the StyleDictionary module
• updated the filterProperties.js to receive the filter parameter only as a function
• moved the logic to handle the case when the filter parameter is an object in the transformConfig method, and added the handling of the case where the filter is a string (supposedly a filter registered via registerFilter, otherwise throw an error)
• updated the documentation accordingly

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[didoo]

@dbanksdesign @chazzmoney before continuing this PR (updating tests and documentation) I need some feedback from you. Have a look at the code, it works but maybe (probably) can be better.

Things I am not 100% sure:

1. I have moved the logic where we convert the filter from object/function to function (and added the case for the string (registered function) from the filterProperties method to the transformConfig one. The reason for that is that in filterProperties I don't have access to the StyleDictionary object (where the filters are registered). I have tried to pass it down, or to use this but without success. Do you think there would be a way to do it? Where is the "correct" location for this logic?
   

   style-dictionary/lib/transform/config.js

   Lines 55 to 70 in b7d1672

   	const ext = {};
   	if (file.filter) {
   	if(typeof file.filter === 'string') {
   	if (dictionary.filter[file.filter]) {
   	ext.filter = dictionary.filter[file.filter];
   	} else {
   	throw new Error('Can\'t find filter: ' + file.filter);
   	}
   	} else if (typeof file.filter === 'object') {
   	ext.filter = _.matches(file.filter);
   	} else if (typeof file.filter === 'function') {
   	ext.filter = file.filter;
   	} else {
   	throw new Error('Filter format not valid: ' + typeof file.filter);
   	}
   	}

2. I have left a comment, about the initialization of the filter object as a property of StyleDictionary:
   

   style-dictionary/lib/register/filter.js

   Line 38 in b7d1672

   this.filter = this.filter || {}; // TODO: why the other registerXyz functions don't need it? where are initialised their objects?

   
   I get why we need to initialise it (templates and formats are always present in SD, filter not necessarily) but I am not sure if there is a place where is better to initialise it. Any better ideas?

[dbanksdesign]

For question 1, you are correct in the placement. transform/config.js is meant to 'hydrate' the config object so it has functions rather than string keys for transforms/formats/actions/etc. Adding filters, this would be the proper place to do it.

For question 2, the initialization happens in the main index.js file:
https://github.com/amzn/style-dictionary/blob/master/index.js#L36
The file that it requires returns an object. So we could just add:

filter: {}

in there and call it a day. Or do we want to include built-in filters? Maybe not if we are trying to distribute/open up the transforms/formats/actions. Sorry for stream of conscious here.

[didoo]

question 1

Cool, I am happy then! :)

question 2

Ahh, now I get it! Now that you have explained it, makes total sense. OK, so we have these options:

1. we leave the "initialisation" in the lib/register/filter.js file, but is an antipattern
2. we add a line in index.js like you suggested (with a comment why is done like this)
3. we create an "empty" module lib/common/filters that returns an empty object, so is consistent with the others (we need to add a comment in the module anyway, and possibly also in the index.js to signal that that is a special module

Probably 2 is the simplest and clearest one. What do you think?

[dbanksdesign]

I like 2 or 3. We can just go with 2, and then if we do add built-in filters in the future we can update it when that happens.

[dbanksdesign]

One tiny tiny thing...

[didoo]

@dbanksdesign wait before merging, I still have to update the documentation :)

[dbanksdesign]

Good call, I might have merged today by accident

[didoo]

@dbanksdesign I have updated the documentation files. Do you think is enough?

[dbanksdesign]

LGTM!

[chazzmoney]

LGTM