A pacman hook for less manual maintenance

[5long]

Ideally a pacman hook for p3wm should work like this:

• Defined as a PostTransaction hook
• Merges .pacnew files non-interactively
• Don't accept the merge if the file can't be cleanly merged
• Leaves backup file even if it's a clean merge
• Save merging transactions to a log file
• Make the hook opt-in / opt-out ? Anyway there must be a way to disable the hook if the user doesn't like it.

Depends on #9.