Add Command Aliases

Git has the feature where we can set aliases for commands. It would be nice if git gud did the same thing. For example: `git gud config alias.c "commit"`

The issue names no files, tests, or entry points. Start by locating how git-gud commands and configuration are handled, then verify that the documented `git gud config alias.c "commit"` example invokes the aliased command and that the alias behavior is covered by tests.

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