Add possibility for github command to accept arbitrary number of arbitrary arguments.

- Also, don't pass kwargs everywhere like crazy.
Properly expand variables in format_deep method of Command
Few GithubCommandRunner fixes:

- Make add_remote_origin runnable from assistants.
- If arguments are specifed but empty, use computed values instead.
- If github variable is not in context, don't fail but fall back to os username.
Add forgotten is_var function to lang