Add option to pick background color preference for Twitch emotes

Twitch has a setting in their URL to adapt emotes for light or dark background. 
Right now I'm using `dark` as a default, but would be neat if the user/dev can pick it for its case.

https://github.com/mkody/twitch-emoticons/blob/f1c98f05dab225a3ca2e07cff399223290af1d26/src/util/Constants.js#L3C85-L3C89