Partial alias support

These changes add support for simple aliases (1:1 mappings between aliases and commands, like `git st` → `git status`). Compound aliases (e.g. `git ac` → `git add -A && git commit`) are still not supported, but it prints an error message letting the user know.

Tests & documentation updates are included.

__Note__: when running the tests, your .gitconfig gets temporarily overwritten. While it should restore the original config afterwards, you may want to back up your .gitconfig in case things go awry.