Support subpaths for http datasources

[hairyhenderson]

I thought I could provide a second argument to datasource to build on the URL for HTTP datasources:

gomplate -d d=http://example.com/ -i '{{ datasource "d" "foo/bar/baz" }}'

Which should effectively access http://example.com/foo/bar/baz.

But, it didn't work!

Now, with this PR, it does.

This also splits the http datasource code into a separate file (datasource.go is getting a mite large)

Signed-off-by: Dave Henderson [email protected]