Added gpgWarnOnFailure

[djspiewak]

Feel free to bikeshed the name! This adds a new key, gpgWarnOnFailure, which is scoped to Zero and publishLocal. In publishLocal it is defaulted to true, and everywhere else it defaults to false. A value of true will override the default behavior and allow publication to succeed even if signing fails. A value of false corresponds to "fail on failure" and will result in the build halting if signing cannot be completed successfully.

Oh I also fixed a previously-undiscovered bug when gpgWarnOnFailure := true and signing fails for not-the-first file. Previously, the warning would report that "Artifacts won't be signed", but in fact some of the signatures would have still been published. The behavior is now consistent with the error message: when any files fail to sign, no signatures are published.

Also as a bonus, I fixed gpg's process output to redirect fully to SBT's logging. This always annoyed me about gpg from within sbt-pgp.

Fixes #3

[jodersky]

thanks for working on this! I'm about to head out to a trip this weekend without any connectivity, but I'll take a more detailed look when I get back

[jodersky]

Thanks, this is really nice! (I also appreciate the typo fixes and best practice recommendations 😄 )

[jodersky]

Released in version 0.2.0, which should be published to maven central when the build completes.