VCS hosting abbreviations

This lets you use abbreviated URLs for templates on common hosting sites, like `gh:audreyr/cookiecutter-pypackage`, as discussed on issue #246. gh expands to Github, and bb to Bitbucket.

I haven't tried to make this configurable by the user at present - that seems rather less useful to me. However, technically it would be fairly straightforward to add those if there's demand.
Why a KeyError? Why not:

```python
template = hosting_abbreviations.get(abbr)
if template is None:
    return input_dir
```