make default protocol customizable

make default protocol customizable

This adds a defcustom to let user choose what default URL type elpher
should assume when no one is explicitly given, instead of hardcoding
gopher as it is now.

It also improves a bit how scheme-less URL are handled: `M-x elpher-go
example.com/foo/bar' is now parsed into host `example.com' and path
`/foo/bar'.