Provide JavaScript-based downloading method

Fill PDF currently operates in one of two post-processing modes (in other words, what it does after the merge into the PDF is complete). This leads to the following possible scenarios:

1. The file is immediately sent to the browser when the Fill PDF Link is requested (leaving the user on the same page).
2. The file is saved to a configured path.
3. If a path is configured and download=1 is passed in the query string, both.

Some workflows, however, require that the user is directed to another page and that the download initiates a couple seconds after that. This may be for tracking purposes, or the client might just want it.

I'd like to support this in Fill PDF. It would need some JavaScript (one possible approach is outlined in http://stackoverflow.com/a/3749395) and for an additional query string argument or menu callback to switch it into this mode.

This is a nice-to-have, but I would welcome a patch enabling this.

I probably want to implement #1517652: [PP-1] Increase redirectability of Fill PDF URLs first though.