Add optional step to create pgp-signed git commits

[patcon]

New to this myself, but seems it would be a nice addition :)

Ref: gratipay/roobot#4

To Do

• add signed tagType to switch
• check for gpg command and throw helpful output if unavailable
• check for git config user.signingkey config value, with helpful output
• pass any remaining git failures forward

[patcon]

This is getting handled in #66 since they're very similar

[patcon]

There are a few errors that can happen. We could just pass the git error forward, or we could do what is probably most of the work and check that:

• which gpg works
• git config user.signingkey is present

[tomkersten]

+1
Even in current state this is a nice addition. The people who are most interested/concerned with signed releases likely will either meet the remaining requirements in the list above...or will know how to address them.

[drublic]

This PR looks good to me. @patcon do you want to work on the other tasks you created yourself or should we merge here and we'll look into the other tasks later?

[patcon]

I'm not actively using grunt-release right now, so it might be awhile if you're waiting on me. 😞

In the interest of getting it in, I'd say merge and reticket anything that still needs doing :)

Thanks for the issue queue gardening btw!