Support using a function instead of a plain string for urlText

[nogweii]

I'd like to expand the syntax used for markdown in the urlText, like by specifying the height & width (an extension to normal markdown):

![file_name)(/imgs/123456789.png =120x60)

Though I don't want to hard-code a default size, and instead customize it based on data returned from the server. My thought is that a function could be set for urlText that will be passed in the data returned from the server. The function is expected to return a string that will be placed literally into the textarea (so if you opt to use the function, you must make sure to include the attachment's name in the string).

[Rovak]

I'll add another hook for it, i think it can be added to the onFileUploadResponse response and add another parameter in which the resulting text can be modified that will be inserted in the textarea

[zilkey]

Here's one way it can be done: #70