Support SSH Include directive

My .ssh/config file uses the Include directive but this is not supported and parsed by import sites feature of WinSCP.

I created a fork and made some edits to the code to make it working.
Can I open a PR directly on the repo? Or are there some steps to do earlier?

I can't find a good contribution guideline.

Thanks for your contribution.
I'll look at it in few days.

Your PR: https://github.com/winscp/winscp/pull/51