Add Elm lexer

[gandarez]

This PR simply adds Elm lexer to get the benefit of its tokenization.

[alecthomas]

I think maybe you're misunderstanding what the embedded/ directory is for. Chroma embeds all of those XML files and internally loads them into the lexer database.

You can see an example of Elm here.

[gandarez]

I see. So the best way is to call lexers.Get("Elm")...?

[alecthomas]

Yes, that's correct.