Feature: Make tgswitch working on windows

[mivalsten]

This PR makes tgswitch working on windows by changing default path. It also allows to have different default path on each OS.

[mivalsten]

Added windows specific CreateSymlink as windows requires admin privilage to create symlinks, so if the program was ran as unprivileged user it would fail no matter what. Windows version instead copies the file to path location.

Not the best solution, but option to symlink as non-admin user only became available in windows 10 and requires customized syscall that is not available in go libraries yet.

[williamoconnorme]

@warrensbox can this PR be reviewed please? tswitch in it's current state is not compatible with Windows due to the download URL not appending .exe file extension to the URL. This results in tgswitch downloading the 404 HTML response page from github

@mivalsten thank you for distributing the built binary in your fork. I've tested it locally and it's working great for me