Support no crawl/no index at ENV level

:fork_and_knife: 

Allow Supermarket to be configured to be indexed and followed by search engines
at the ENV level. Set the NO_CRAWL in .env to true if you want to disable
indexing and following of the site at the search engine level.

Some interesting reading: http://googlewebmastercentral.blogspot.com/2007/03/using-robots-meta-tag.html
should `"true"` be `'true'`? I'm not sure if rubocop will be ok with this or not.