Add upload and download button

[flafflar]

Currently, the only way to minify a Javascript file with this tool is to open it, copy everything and paste them in the website. It would be much more easy if there existed an upload button for uploading Javascript files.

Also, to store the minified code, copy/pasting is required again, so a download button that downloads the minified Javascript file would also make things easier.

[Skalman]

@the-wulv, I agree, this seems like a good addition. Feel free send a pull request with this feature.