Add support for arbitrary domains

[mohd-akram]

Closes #2.

Tested with go run go2port.go -d get github.com/caddyserver/caddy v2.1.1.

[amake]

Thank you for your contribution.

I had to make various changes, most notably disabling the special handling of go.googlesource.com. The reason is that in my testing go.googlesource.com serves a different (i.e. checksum doesn't match) tarball each time you download. Such a service can't reliably used as a host for distfiles in MacPorts.

[mohd-akram]

Thanks for merging! Issue for reference - google/gitiles#84.

[amake]

Ah, thanks very much for that 👍