add support for outputing json

while gulp-livescript pass options to livescript such as "bare", LiveScript.compile doesn't support "json" option for json generation. The json generation feature is wrapped in command.js:CompileScript function inside LiveScript package as a command line option.

Yet sometimes we might need to compile livescript into json, so I adopt the code in command.js and patch gulp-livescript so that one can add an option "json" to generate json file from livescript, for example:

    gulp.task 'json', ->
      gulp.src 'json/*.ls'
        .pipe gulp-plumber!
        .pipe gulp-livescript({+json,+bare}).on 'error', gutil.log
        .pipe gulp.dest "json/"