feature: add label property to DiscreteFilter

[okdistribute]

fixes #68

@GoGoCarl is this what you were thinking?

[GoGoCarl]

@Karissa Not really, but I think I did not completely explain my full intentions in the ticket as I had discussed this with Gregor before.

I've updated the ticket to include a screenshot of what I'm talking about. So, the label I'm looking to customize is for the value of the field:

https://github.com/digidem/react-mapfilter/pull/82/files#diff-e58a7d48fe0412723727ccaef23c2184R154

That said, it WOULD be nice to be able to customize the entire filter heading as well as you have done here, but I think it would be best served if this were possible to do at a translation level. There's a translations map that gets initialized in MapFilter here:

https://github.com/digidem/react-mapfilter/blob/next/src/containers/MapFilter.js#L28-L43

It would be cool to be able to hook into that mapping somehow. Then, I could add my own translations for things like field_key.happening to have it translate in different languages.

[okdistribute]

What does your imagined API interface to hook into the translations object look like here?

[gmaclennan]

How about we allow translations as a prop, and deep-merge with the translations map? We could just have field_key translations and field_value translations, but I don't see harm in allowing all translations to be optionally overwritten. That way the component consumer does not need to wait for translations to be merged into react-mapfilter.

[GoGoCarl]

@Karissa I think what @gmaclennan described above would work well.

[okdistribute]

@GoGoCarl ok this should work now!