Implement scale

[dahoo]

The usage help mentions --transform scale(x y), but it doesn't work, because there's no scale method. 😉

/home/daniel/.nvm/versions/node/v5.5.0/lib/node_modules/meshlib/bin/cli.js:174
                modelChain = modelChain[transformation.type](
                                                            ^

TypeError: modelChain[transformation.type] is not a function
    at /home/daniel/.nvm/versions/node/v5.5.0/lib/node_modules/meshlib/bin/cli.js:174:49
    at Array.forEach (native)
    at ModelBuilder.<anonymous> (/home/daniel/.nvm/versions/node/v5.5.0/lib/node_modules/meshlib/bin/cli.js:173:22)
    at emitOne (events.js:90:13)
    at ModelBuilder.emit (events.js:182:7)
    at ModelBuilder.<anonymous> (/home/daniel/.nvm/versions/node/v5.5.0/lib/node_modules/meshlib/build/ModelBuilder.js:27:21)
    at emitNone (events.js:85:20)
    at ModelBuilder.emit (events.js:179:7)
    at finishMaybe (_stream_writable.js:468:14)
    at endWritable (_stream_writable.js:478:3)

There could also be a better error message if you're using an argument that is not supported.