add configurable ability to *always* gpg sign releases & hotfixes

It's easy to forget to use `-S` when using `release finish` or `hotfix finish`.

This adds configurable auto-signing via a new git config setting: `gitflow.always-sign`

 If this value is:

* __true__: *Both* releases and hotfixes will always be signed when finished.
* __hotfix__: All hotfixes will be signed. Releases will only be signed if `-S` is passed on the command line, as per the default.
* __release__: All releases will be signed. Hotfixes will only be signed if `-S` is passed on the command line, as per the default.
* __false__: Auto-signing is disabled. _This is the default behavior if_ gitflow.always-sign _is unset._