Support serving all fonts

[jjant]

Currently only ttf fonts can be served (because of https://github.com/elm-lang/elm-reactor/blob/master/src/backend/Main.hs#L285), mime types are missing for otf, woff, woff2, etc. So elm-reactor just doesn't serve them.

I'm not too sure why that mime type boilerplate is there anyway, but if you want to keep it, you should add support for more formats.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.