When used, pyyaml generates *.pyc files, which will cause the submodule
to appear as dirty. This will bubble all the way up to any parent
repository that include Dotbot as a submodule. This change makes it so
that changes in the pyyaml repository do not cause the parent repository
to appear to have modified files.
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.
