Add getValidThreadColors

[AstroCB]

Adds a function to get accepted values for changeThreadColor and validation within changeThreadColor to prevent undefined behavior for non-supported colors as per #506.

[Schmavery]

Thanks for contributing some more changes :)
Is there any reason you're not just exposing the data directly? As in:

module.exports = function(defaultFuncs, api, ctx) {
    return ["#44bec7", "#ffc300", "#fa3c4c", "#d696bb", "#6699cc", ...];
};

I'm also wondering if it wouldn't be more readable if we actually made it a map of the human-readable names exposed by facebook to the colors. The python clone of this api does this. i.e.

{
    MessengerBlue = ''
    Viking = '#44bec7'
    GoldenPoppy = '#ffc300'
    RadicalRed = '#fa3c4c'
    Shocking = '#d696bb
    ...
}

Might end up with more readable code and make it more possible for us to add/adapt/remove colors as time goes on (though I'll admit the names are... strange at best).

Just seems like it would be nicer to write/read
api.threadColors.RadicalRed than api.getValidThreadColors()[4].

[AstroCB]

Ah, I haven't seen any constant (non-functional) data exposed via the API so I wasn't sure if that would be against the design you guys have set up.

Are those names the official ones exposed by Facebook? I haven't seen them anywhere.

[AstroCB]

I switched it to a dictionary using those names; the code should definitely be more readable that way.

I also renamed it to validThreadColors before I saw your note on usage, but I can change it to just threadColors if that's more idiomatic.

[bsansouci]

Seems good to me though I'd change the docs to say that it is an object rather than it returns an object.

[Schmavery]

Maybe we can move the note about "Due to Facebook backend changes, the thread color can no longer be set to an arbitrary hex value." to the CHANGELOG.md file and put a link there to this PR? (See the other entries for an example)

[Schmavery]

I feel like the validity is implied by the fact that we're providing the colors, so just threadColors is probably more concise. Thanks!

[AstroCB]

Done.

[Schmavery]

Seems good, thanks a lot.