Router needs to return the correct mime-type for public files instead of `text/plain`

Now that Swifton defines its own ContentTypes in Response.swift we need to add the missing mime-types in order to return the correct content-type. Returning a text/plain type causes errors on certain browsers that will only parse a file if it has the expected mime-type:

Did not parse stylesheet at 'http://localhost:8080/css/style.css' because non CSS MIME types are not allowed in strict mode.

Here's what needed:

A way to find the correct mime-type based on file extension (which we already have in MimeType.swift
To add all of these to the ContentType enum in Response.swift
To return the correct one in Router.swift:38

sauliusgrigaitis / Swifton

Router needs to return the correct mime-type for public files instead of `text/plain` #41