add optional jarPath setting - complete path to .jar file

[himdel]

adds optional jarPath setting - complete path to .jar file

if present, it should be a complete path (including filename) to the jar file
if not, the original closurePath + '/build/compiler.jar' is used

example (needed in debian):

        'closure-compiler': {
            frontend: {
                closurePath: '/usr/bin/closure-compiler',
                jarPath: '/usr/share/java/closure-compiler.jar',
                js: 'js/*.js',
                ...
            }
        }

it is an (IMHO simpler) alternative to pull request #36

[himdel]

Closing as there's been no activity for years.