Analyze changes in git index using "-i" option

Hi,

I could not find a way to run pronto on the staging area so I added `-i` option.

What do you think?

Note: I couldn't find specs neither for `Pronto::Git::Repository` nor for `Pronto::CLI` so I did not add more specs :( Sorry for that!

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After this commit you can run pronto on your current
staging area (git index).

This way you can analyze _before_ you commit.

Usage:

    pronto run -i