Add possibilty to mount folders using reverse mode

[couraudt]

I wonder if it's possible to add folder mounting using reverse mode?

Thank you for this port of gnome-encfs for gocryptfs!

[cjermain]

@couraudt, that is a great suggestion. I will add it to the project tracker and try to find time to work on it. Pull requests are definitely welcome.

[couraudt]

Sure why not, I don't know go that much but you've done all the job, I can try doing just part this in a PR using your code as example.