Linux support

[nomuus]

This updates the code to handle paths and file operations in a more OS-independent manner. I deliberately did not refactor too deeply and tried to keep the general style intact. It's not the most elegant per se, but it should allow compiling payloads on a Linux host.

It adds a new argument to optionally fix the project file's target framework moniker which makes support for Linux easier than modifying them manually (the available argument choices are taken directly from the Microsoft site). There may be other means of achieving this end directly via dotnet core but I've not investigated any further.

More details are in the Linux.md file.

Disclaimer... I've not extensively tested as this was a modification done this afternoon in order to suit my use case.

[t3hbb]

Hi @nomuus

Thank you so much for this! I haven't had a chance to go over it properly (looks amazing at first glance) but wanted to let you know I am looking at it and will merge your changes along with some other updates into the next version (have added some other methods / frameworks and did a little tidying). Really appreciate the effort and contribution. I will of course credit you (unless you don't want me to) for the updates.