Support --directory in abbreviation definitions

[exhuma]

This PR adds support to specify a "directory" in an abbreviation (as in the --directory CLI argument).

In order to support this while still being backwards compatible, the code needed a slight adaptation in how expansions are executed.

When this is merged, a user can write abbreviations in two variants:

• simple: A simple string. This is the same behaviour as currently exists in cookie-cutter
• advanced: A mapping. This new format allows to add the directory option.

This change also opens up the possibility to add more keys to abbreviations, opening up the doors to more advanced abbreviation handling, while still keeping backwards compatibility.

[exhuma]

Updated with "main" again and the diff still looks clean. Any update/feedback on this?

[liortct]

Hi @exhuma

We started working on a new community fork of the Cookiecutter project, named Cookieninja.
The fork is found at: https://github.com/cookieninja-generator/cookieninja

We have re-opened and merged your PR in the Cookieninja project at: cookieninja-generator/cookieninja#64

We're trying to bring the life back to this amazing project.
We are committed to reviewing and merging each PR in up to 2 weeks from the day it's opened (if there are no issues with it).
We'd really like you to join the fork, star it and contribute to it!

Thank you :)