GPG no-tty helper script and options

[asfaltboy]

As in #596, this change documents a proper GPG setup on tty-enabled OS (Linux/MacOS), and correct credentials docs. This commit also adds the global_pre_flags setting to allow specifying options BEFORE the command, such as the "-c" git per-command config option.

The GPG prompt will only work in Sublime if user sets up git config with the options:

gpg.program=./scripts/stgpg.sh
commit.gpgsign=true

or instead of last one, passes the -S flag to commit command. So, I added the options in the GitSavvy settings comments.

TODO / Discuss:

• Figure how to use the script file from the packaged zip. Maybe create the file in the User folder on launch?
• Should we do this by default? Or only if user specifies an option? Or leave it in settings / docs?
• gpg-agent could freeze (happened to me while importing a key), breaking the whole commit flow, until it's killed. Should we take responsibility for this and warn users?

[randy3k]

I guess we could merge this PR first and evaluate its usefulness.

[randy3k]

For the record, I use MacGPG and its gpg works in non tty software.

[asfaltboy]

I think without at least documenting it, it won't be of much use. Sorry I didn't get a chance to