Add format for {{index}}

[Kerruba]

This pull request add the possibility to define a number format to use with {{index}}.
Couple of test cases have been created

[75lb]

good patch, thanks! Useful feature, i agree it should be possible to control what {{index}} is replaced with.. will think about this for a future release.

[75lb]

Renamer has been completely rewritten so I can no longer cleanly apply this patch. However, it's all implemented (using printf as in your PR) in the next version with the --index-format option.

Install the prerelease:

$ npm install -g renamer@next