Add srcMaxSize option to viewers

[wabson]

Similar to the built-in Image plugin, the Media Viewers plugins should support checking the file size of the source file, allowing administrators to configure a maximum file size beyond which the viewer will not be used.

[wabson]

Example code from Image.js

var srcMaxSize = this.attributes.srcMaxSize;
if (!this.attributes.src && srcMaxSize.match(/^\d+$/) && this.wp.options.size > parseInt(srcMaxSize))
{
    // File is too big
}