Update local dotfiles when running `fresh update`

You can use `fresh update --local` to just update your local dotfiles.
Move repo updating code into its own function
Include local repo updates in the update log
Merge branch 'update-local'

Closes #54

* update-local:
  Include local repo updates in the update log
  Move repo updating code into its own function
  Update local dotfiles when running `fresh update`