Added persistence option

[phux]

At least for me it makes sense to not lose the scratch window contents when closing vim. Also includes (optional) per-project scratch window contents when option set to relative path.

What do you think?

[mtth]

Thanks for the pull request, I like this a lot actually.

(A few minor comments inline and we should be good to go.)

[phux]

Sure, there you go. :)

[mtth]

Awesome, thanks @phux!