Automatically pickup and update existing templates

[fregante]

Running $ ghat without any sources should cause any existing templates to be updated. The update command for each is included in full in the header of each file. I think it can just load every file, parse the command, deduplicate any commands (because a command can generate multiple files, and then run them in parallel by calling itself)

This would enable users to update the templates by just calling ghat, without specifying each one separately.