initial attempt to allow user-provided handlebars

- pass a user-provided hb instance via plugin like
    .plug(LocatorHandlebars.yui({handlebars: handlebars}))
- allow that to override the locally installed hb
try app.yui.applyGroupConfig
fix core.js tests
add test for default & custom hb getting applied; whsp
add tests for HB extensions config
add HB & extensions as config options
rm custom HB example
rm outdated example app dependency
change '-yui' version-substring assertion to object comparison
Merge pull request #10 from isao/user-handlebars

support custom handlebars thru plugin config