Option to jade cli to customize client-side template function names

A patch to add the ability to customize the name the template function is exposed as. Based on the comments in #1354 . Includes Command line and API options with unit tests.

`jade.js --client --no-debug -e myTemplate template.jade`
or
`jade.js --client --no-debug --expose myTemplate template.jade`
or
`jade.compileClient('p foo\np #{bar}', {compileDebug: false, expose: 'myTemplate'});`