Allow user-defined aliases for cookiecutters

Rather than having separate "aliases" and "prefixes", how about this? Just have "abbreviations". If the ```input_dir``` is either an exact match for an abbreviation, or an abbreviation followed by a ```:``` followed by a suffix, then replace the ```input_dir``` with the expansion.

That will give some slightly odd edge cases (assume the defaults, plus an alias ```xx: my_cookiecutter```):

```
gh -> https://github.com/.git        (no suffix, so {0} replaced with nothing)
xx:foo -> my_cookiecutter            (no {0}, so suffix ignored)
```

I don't think these are bad enough to be worth worrying about.

I think this is easier to explain and understand than the current version, so I'm tempted to go with it. Would you agree?