Added various options, https support

[typpo]

I refactored things into an ImageProxy class that you can use like so:

  var myproxy = new ImageProxy({
    validMimeTypes: ['image/png'],
    hostsWhitelist: ['mysite.com', 'images.mysite.com'],
    headers: {'Cache-Control': 'max-age=31536000'},
  });

  // mount the proxy at '/proxy'
  app.use( '/proxy', myproxy.requestHandler );

We accept the following options:

• validMimeTypes - defaults to jpg/jpeg, png, gif
• hostsWhitelist - defaults to the same host only
• headers - default to no additional headers besides content-type

I also added support for proxying images over https.

I realize that these changes are not backwards compatible so I understand if you'd prefer to keep things the old way. I needed these features, so I went ahead and built them anyway. Thank you for starting this project!

[jpmckinney]

+1 for hostsWhitelist

[jpmckinney]

FYI I ended up writing my own tiny app: https://github.com/opennorth/image-proxy/blob/master/web.js

It does the tempfile management more sanely by taking full advantage of gm's input stream features.