Give the option of storing values outside config.cson

Since these settings are mostly machine-specific and some people share
their Atom configuration between machines through various mechanisms,
this change makes it possible to store the Remember Window values
outside the main configuration file. The default is to store the values
as they always have been so that there is no change unless people
specifically make the change.
Merge pull request #1 from lee-dohm/store-outside-main-config

Give the option of storing values outside config.cson