Implements multi-file upload and download

[Patches72790]

This PR affects the following two issues tracked relating to multi-file upload/download in an ssh session. I implemented a similar approach to qcha0 (you'll have to take it on faith that this is my own independent work, but I was obviously influenced by some of their design decisions). However, since they don't seem to have been active for some time, I decided to go ahead and submit a PR for this. I have been using your go library (thank you by the way for implementing this great tool!) in one of my projects, and it would be very nice to have this feature as it simplifies my project's implementation significantly. The change mainly consists of just creating a new session with the ssh client each time you upload/download a file rather than keeping around the ssh session for the lifetime of the struct.

I added a test that uploads two files and downloads them subsequently to your tests folder, but I can add a couple more depending on what other edge cases I should cover.

This PR should close both issues #59 and #60 in your issue tracking.

Let me know if you need me to change anything or add anymore test coverage.

Thanks!

[bramvdbogaerde]

Thanks for your contributions. Looks good to me and solves a long-standing feature request. I am happy to merge this.