FileServer: add simple content type detection

[paluh]

Sets content type based on file extension predefined in static map in FileServer.purs.

[paluh]

This PR slightly extends behavior of FileServer. It introduces mapping between file extension and ContentType header value. Mapping was taken from:

https://www.askapache.com/htaccess/

and

https://htaccess.wordpress.com/2008/07/16/sending-correct-content-type-headers-with-htaccess/

Merge?

[paluh]

I can also use your suggestion from #57 and add this content type mapping as a parameter to server options record (together with userProvidedHeaders from this other PR). What do you think?

[owickstrom]

Looks good @paluh ! I say merge this, and then do the header user function separately. 👍

[paluh]

Thanks!