Add extension parameter type

[ahopkins]

Resolves #27

This PR adds file extension routing.

definition	example	filename	extension
<file:ext>	page.txt	"page"	"txt"
<file:ext=jpg>	cat.jpg	"cat"	"jpg"
<file:ext=jpg|png|gif|svg>	cat.jpg	"cat"	"jpg"
<file=int:ext>	123.txt	123	"txt"
<file=int:ext=jpg|png|gif|svg>	123.svg	123	"svg"
<file=float:ext=tar.gz>	3.14.tar.gz	3.14	"tar.gz"

This implementation will not support the path parameter type: <file=path:ext>. That is a whole different beast and should be handled in its own PR. This feature will be released as "beta", so we can work on getting support for path in the future, or to decide that we will not support it.