add support for shorter aliases of commands

for example, instead of typing:

```
git hub request create
```

make it possible to type:

```
git hub pr create
git hub pr c
git hub mr c
git hub r c
```

(a suggestion from @bbouvier)

Start by locating the CLI command parsing and dispatch entry point for `git hub request create`. Compare how the example aliases (`pr`, `mr`, `r`, and `c`) should map to existing commands. Done means the shown shorter forms work consistently without breaking the full command.

Written by the indexing model from the issue text.