Consider supporting Groovy's ConfigSlurper in main JBake configuration file

[jonbullock]

See: http://docs.groovy-lang.org/latest/html/gapi/groovy/util/ConfigSlurper.html

Suggested via mailing list: https://groups.google.com/forum/#!topic/jbake-dev/UgRcgRTYjUQ

May relate to #379 too.