Persist buffer

[CatTail]

When close the current vim process and create another one by open other file, buffer should not be cleared unless gS entered.

[mtth]

The scratch window is meant to be ephemeral by design.

Support for optionally backing it up using a file would still be a nice plus to have (this would probably raise some issues when there are multiple Vim processes running though). I'd be happy to include this feature if you'd like to submit a pull request.