BEMTREE: introduce js(), mix(), mods() and elemMods() modes

[miripiruni]

After discussion in #332 we decided to introduce new modes in BEMTREE.

We dicieded to introduce only data related modes (not HTML related).

[miripiruni]

It is useful to @collapsus for example

[miripiruni]

The plan: almost no difference in modes with BEMTREE and BEMHTML, only output format difference (BEMJSON, HTML).

[tadatuta]

Let me try once again: I don't think these mods are useful enough for BEMTREE and moreover they are kinda harmful: user just should not set such things like attributes, classes, etc in BEMTREE — it's all about HTML. And extra modes will make BEMTREE slower.

Are there really strong arguments for it?

[vithar]

I agree with @tadatuta

[tadatuta]

We talked with @collapsus and decided it should be enough to add just js() mode which is widely used for blocks with models, i.e. the list of modes in BEMTREE should be following:

• def() + extend(), replace() and wrap() helpers
• content()
• js()

[miripiruni]

@tadatuta can you show example of js() usage?

[collapsus]

yet in the compact syntax
not for models

    js: {
        var currenciesDescription = this.data.currencies_description,
            currencies  = {};

        Object.keys(currenciesDescription).map(function(key) {
            currencies[key] = currenciesDescription[key].full_name;
        });

        return {
            currencies: currencies
        };
    }

for models

    js: {
        modelName: 'm-autopay-settings',
        modelData: this.modelData
    }