When used, pyyaml generates *.pyc files, which will cause it to
appear as dirty. This will bubble all the way up to the owning
dotfile repository. This change should make it so that running
a dotfile install script will not cause any repo-level changes
by default.
Add support for YAML format configuration files. In addition, this
commit adds instructions about YAML config files to the README, and it
also changes the README to encourage use of YAML instead of JSON.
