Allow phpcs search path patterns to be overridden in config

I needed to change the patterns for phpcs to check, mostly due to certain modules' Features exports not conforming to the standard(?) naming pattern of `*.features.*inc`. This adds support for that in the config, using the existing directories as the defaults. 

I also found that although the `css` extension is not included in the options for grunt-contrib-phpcs, css files were still being analyzed due to the globbing pattern in the defaults. This might be a separate issue (perhaps in the phpcs plugin) but this change allows me to exclude them by overriding for now.