`Args.InsertParam` panics about index out of bound if the index is larger than param size. These changes reset the index to param size for such case. This is equivalent to appending new items to the end.

This fixes #812.

Start reading the `Head.Repo.Name` property from API payload to
determine the name of the forked repo which might not match the name of
the base repo.

- Use a hash to lookup for commands.
- Add more tests for the command and runner execution.

Since https://github.com/github/hub/compare/31a85e519e0a080dd10ce21339f806a9a0877766...master

Some tests are ignored either because they don't make sense or WIP