Add support for "concatCSS" option in config

Versions of IE prior to 10 had a relatively short 4095 selector limit which may inadvertently be hit as a result of concatenating all the layers CSS documents together. Added option to disable concatenation.
Updating documentation for #146

Updated the documentation for the 'concatCSS' feature introduced to work around the 4095 selector limit in IE 9 and below.
Merge branch 'fix-ie-4095-limit' of github.com:jardilio/require-css
adjust IESelectorLimit option