Add git submodule management scripts

Inspired by the frequent creation of tools to get around using git
submodules. Instead, give users a single command to update a submodule.

Broken down into subcommands so if you're developing a submodule, you
can use git-sub-commit-changelog, but if you're just following it, you
can use git-sub-ff-and-commit.

Includes some commands from my .gitconfig that are useful within these
commands.
Clarify usage and comment on license