update checker

[justone]

A command dfm checkupdate that you can put in your .bash_profile (or zsh equiv) that will only check for updates once a day and will have one of the following outputs:

• Nothing - if there are no updates
• A list of updates (log summary) and "you have updates, run dfm umi"

Alternatively, it could just merge the changes in if configured.

It should:

• complain if there are local changes and there are changes to pull in
• refuse to merge changes in if there are local changes

[justone]

We don't want it to run if you are using mass SSH or ansible, so it shouldn't run if there's no terminal attached.

[justone]

For configuration, how about a ".dfmrc" that goes in the root of the dotfiles directory (and is not installed), with options to control this:

• update frequency
• behavior if there are changes to merge

[justone]

Or, call the file .dfmconfig and use git config internally to manage it. No need for a new file format and we already know git will be there.