add new optional convertValue property to FormsyText

[ulich]

When using a FormsyText with type="number" for example, the controlled value is
still a string. By specifying a convertValue function, you can convert the value to a number,
before it gets passed to formsy's setValue() for example.

Another example: a user enters the numbers of seconds, and internally it is converted to milliseconds

<FormsyText name="millis" value={millis / 1000} convertValue={(v) => Number(v) * 1000}` />

The whole thing can be probably applied to a wide range of use cases

[ulich]

thanks! when do you plan to create a new release?

[ryanblakeley]

It's out now, v0.6.1.

[ryanblakeley]

Hey just want to note thanks for this PR. I'm putting it to use already!

It seems like it might be good to force the conversion to happen before sending the value into validations. Do you have any thoughts on that? Would there be a reason to keep those operations separate? Right now I'm duplicating some conversion logic in my convertValue prop and a custom validation function. My first thought it that the conversion should happen, then validate the output. Is there a reason that would not be a good idea?

[ulich]

Sounds sane yeah. I dont see any issues with that 👍