Allow custom session parameters

[L3P3]

As you said in #402, its easy to let users open a picture in miniPaint directly.
My use case is my textures manager for my game. I want users to be able to edit their texture or open another one as a template and then be able to have it uploaded to my server.
https://l3p3.de/svr/minicraft/textures.html
I already added an open in miniPaint link. 👍
Now, instead of forcing the user to download the file and upload it to my form or copy datauri to the clipboard, I would like to have a custom primary save option that is doing a post request to a url I can specify.

Example

https://viliusle.github.io/miniPaint/?image=https://l3p3.de/static/minicraft/textures/1.original.png&saveTo=https://l3p3.de/svr/minicraft/textures-save.api
Would cause miniPaint to have the option "Save" in the file menu and when the user clicks it, a post request is done to above url containing the image. Maybe besides the saveTo param, also have the new save option in file menu "Save to post api".

Thanks, nice project.