Add sign-off feature to git commit

[sagikazarmark]

Enable via configuration:

git:
  commit:
    signOff: true

Resubmission of #1031
Fixes #505

To answer the question that came up previously: no, signing-off does not require password entry.

The implementation in this PR is similar to #1031, but I wonder if it would be better to move the signoff flag directly to GitCommand.CommitCmdStr: it has access to the configuration and the signoff flag should probably always be passed to the git commit command. Let me know what you think and I rework the PR if necessary.

[Mephistophiles]

How about adding parameters to the config for the 'git commit' command, like that commitArgs, commitAmendArgs? It would be useful for me to also forward the -v option at rename the commit.

[jesseduffield]

Nice work @sagikazarmark!

@Mephistophiles I think such a change could be orthogonal to this one i.e. if the user has set some boolean flags, we'll add those, but we could also allow passing a string. I prefer the boolean approach for now if only because it makes it easier down the line if we want to start using something like go-git under the hood to run the git commands. But if an issue doesn't already exist for such a feature, feel free to create one :)