Add fonts mime type mapping to elm reactor

[albertdahlin]

Fonts have their proper mime types since Feb 2017.

https://www.iana.org/assignments/media-types/media-types.xhtml#font

This fixes a problem when trying to use custom fonts hosted locally with elm reactor, for example using @font-face

For example

@font-face {
  font-family: 'Source Code Pro';
  font-style: normal;
  font-weight: 400;
  src: local('Source Code Pro'), local('SourceCodePro-Regular'), url(/source-code-pro.woff2) format('woff2');
}

does not work with elm reactor.

When the font file is loaded it will be wrapped in a HTML document by the elm reactor server (at least in 0.19.0).

[evancz]

I added them in 536d50c, but with two variations:

1. I kept the list alphabetical by extension. I don't know if that is ideal, but it seems good to stick to it and decide on a different order explicitly.
2. This revealed that there was already an entry for .ttf but it looks like it needed to change from "application/x-font-truetype" to "font/ttf" based on the IANA documents you shared.

Let me know if everything looks alright in the patch, and the changes should be available with the next release of Elm.