Added option for left & right delimiters

To prevent from “{{“, “}}” clashes between go’s template and
others(handlebars, angularjs also uses them).
Fix for the comment
Added a specific struct('Delims') to set template variable delimiters
Updated README with 'Delims' usage
Added test for 'Delims'
Merge branch 'master' of github.com:savruk/martini-contrib into savruk-master
Cleaned up Delims docs. Formatting
Changed Delim to Delims
Updated readme