Add possibility to run a pre-process tool

[SicroAtGit]

Excerpts from a discussion in issue #246 in chronological order:

#246 (comment) from @SicroAtGit:

I think a plugin system is too much for the tool. Perhaps it would be good if a pre-process tool could be set in the Flameshot settings, which would then be executed before saving or uploading the screenshot. During this time Flameshot provides the screenshot in a temporary image file, waits until the pre-process tool has terminated and then takes the temporary image file modified by the pre-process tool for the further processing (uploading or saving). This variant would also save having to rewrite functions for uploading and adding an entry to the upload history, since Flameshot does this as before.

#246 (comment) from @mmahmoudian:

Having Flameshot running custom scripts can be nice and perhaps easy to implement, but portability to other OSs might become a problem (i.e Windows). But perhaps it can be somehow managed. It should be discussed in details before we move to coding it.

What I like is that we can have a userscripts section that users can place in a folder and Flameshot can call them.

#246 (comment) from @SicroAtGit:

I think a good cross-platform solution would be to have it not run scripts, but run executables. This way you can write the preprocess tools in any programming language (Python, the C languages, Nim, Rust, etc.). When a Python script is to be executed, the interpreter is executed with the script as parameter: python script.py.

Would be nice to hear what others think about it.