Allow configuration of iframe src attribute

[ghost]

Not sure if you have an alternate thought on how to solve this problem, but the change is simple enough that kicking it off with a pull request makes sense to me.

Inspiration:

In my org, we're planning to ship with Content Security Policy enabled.

This poses a minor problem with the iframe transport, which uses src="javascript:false;", leading to the following error (in Firefox, Chrome gives a similar error):

Content Security Policy: Directive inline script base restriction violated

Proposed Solution:

Allow the user of the iframe transport to configure the iframe's src attribute. Default to the old behavior, javascript:false;.

In my org, all the browsers we plan on supporting handle about:blank, so we could go with that, fixing the issue and preventing any error from getting logged.

[blueimp]

Thanks for your contribution! 👍
I've refactored your code changes slightly and renamed the new option to initialIframeSrc:
https://github.com/blueimp/jQuery-File-Upload/wiki/Options#initialiframesrc