Traverse into symlink directories, take 2

Continuing the work of @geoffb from https://github.com/atom/fuzzy-finder/pull/13 (thanks! :metal:).

As suggested in https://github.com/atom/fuzzy-finder/pull/13#issuecomment-36789958, this adds a config option to the package which enables/disables the traverse-into-symlink-folders behavior. By default, this option is disabled, to match the current behavior of the package.

cc @geoffb @kevinsawicki @probablycorey