Fixes #208

fixes #218, fixes #194

Fixes #120

Enterprise installations don't have a separate subdomain for API
requests such as "api.github.com", but their API endpoints start with
"/api/v3" path.

Fixes #201

Fixes #184

Fixes #187

Fixes #180

Fixes #177

Fixes #169

It doesn't add a remote anymore, as it isn't needed.

It does what GitHub's "Merge pull request" button does:
merges the pull request with a nice commit message that
includes the pull request ID and title. Like `hub checkout`,
it creates a remote, but it does not create a local branch.

This fixes it twice: Use /bin/bash to guarantee same shell on all OS,
and use standard = operator for conditionals.

Replace all dashes to underscores, not just the first one.

Fixes #175