Check if script already running, make POSIX compliant, always pull push with -p

[haoadoreorange]

• Check if another instance of script is already running. Option -a to bypass.
• Also added -m option for specifying commit message used for all commits.
• Only pull if -p is true (didn't make sense to me why would we want to pull if not pushing). Also with -p, now the script always pull push, since if there's no new commit then nothing changes, but if there was commit before the script run then it will be pushed, which was not the case before.
• Option -e to make a final commit/push upon exit.