added a way to selectively upload and delete files from shopify

The upload/delete functionality relies on the watch event and does not integrate well with other tasks (such as copying files from another directory to the shopify theme (through ```grunt-contrib-copy```), less compiling, image compressions and other).

I have added the ```shopifyupload``` and ```shopifydelete``` multi tasks that take as ```src``` config parameter the lists of files for upload/deletion. This config can further be changed on the watch event, which is triggered before tasks are being executed (this is the recommended way of tinkering with the watch tasks, as per ```grunt-contrib-watch``` documentation). This setup is more compatible and integrates better (although not ideal) with other tasks.