Added re-signing of existing signed files.

[hlein]

When editing an existing file, check if the source file was signed, and if so, enable signing when we save it.

When writing out a file, moved the "should we sign" check so that it always happens. When saving an existing file, b:GPGOptions exists already, so g:GPGPreferSign was not being checked previously.

This partially addresses https://github.com/jamessan/vim-gnupg/issues/34

If we wanted to support selecting between different private keys to sign with, then checking which key had signed a file and looking for a corresponding private key in our keyring could be done in the --list-packets check. But that is another future issue & PR.

[jamessan]

Do you intend to update the PR (and preferable GPG sign the commit) or do you want me to just make the final changes?

[hlein]

Do you intend to update the PR (and preferable GPG sign the commit) or do you want me to just make the final changes?

Oh, please go ahead and make the changes if you have the time. I was muddling my way through the code, you will do it right the first time ;)

Thanks!