add `%appdata%\proxinject` to PATH when installing

[soredake]

Otherwise you can't use cli version without specifying full path or adding it to PATH manually.

[PragmaTwice]

There are some limitations in NSIS for string manipulation so that it is hard to append it to PATH.

But it seems this package can do it. I will try it when I'm free. But feel free to do it by yourself if you're interested.

Refer to https://github.com/PragmaTwice/proxinject/blob/master/setup.nsi