rename `changed_files` config setting to `changed_proc`

This is prep for having a non-singleton config handler.  This renames
the `changed_files` setting to make it be named more consistent with
the other proc settings.

This also moves the config class to its own file, again, in prep for
a non-singleton config.
Merge pull request #165 from redding/kr-changed-proc

rename `changed_files` config setting to `changed_proc`