Add dependencies from the CLI

Something like:

1. `gitman add https://github.com/foo/bar`
2. `gitman add https://github.com/foo/bar@abc123`

The use of `@` to specify `rev` matches the `lock` command, updated by https://github.com/jacebrowning/gitman/issues/98.

Start by tracing the existing CLI implementation of the lock command and review issue #98 for how revisions are represented. Add an `add` command that accepts a repository URL with an optional `@rev`, and verify that both example forms create the intended dependency.

Written by the indexing model from the issue text.