Allow the github subcommands to be invoked directly.

[vivekashok1221]

The usage for the github command is currently as follows: !github subcommand. Trying to directly invoke the subcommand like !subcommand shows no output.

For example, !github issue 23 python/cpython. A usage such as !issue 23 python/cpython is currently not supported. It would be cool to support such a usage.

This issue can be addressed in the same PR as issue #84.