Add fetlang to github linguist

[spacekookie]

More of a meta-issue than anything to do with the language. But you can add languages to the github language detection (called linguist) here: https://github.com/github/linguist

I think you can even assign a colour value to it (otherwise it's just grey) but I'm not 100% sure about that.

Could be cool to have github recognise the .fet files correctly 😄

[yochem]

It's only possible if the file extension is used in "hundreds of repositories before supporting them in Linguist." (source). Guess we have to start coding in fet :)