Feature: Hot exit (persist unsaved changes on quit)

[bryphe]

This is related to the following issues:

Prerequisities:
#610
#600

Once our 'quit' lifecycle is refined (ie, we're handling unsaved changes correctly...) - I'd like to add another layer in terms of "hot exit".

Hot exit would work as follows:

• Persist the swapfile for active, unsaved buffers
• Persist the undofile for all open buffers for persistent undo

On quit, we'd save all the relevant swap files in the Onivim folder, and reference them via a configuration file (workspace.json). When we load, we'd check and see if there was an active workspace.json, activate it (ie, populate the splits as before), and then load the swapfiles and undofiles for buffers as we load them.

Essentially - it'd just act as a convenient, automatic way to reuse the swapfiles & undofiles that Vim already has for this sort of persistence.

Some open questions:

• What new APIs would we need for libvim? Would we want an explicit API to save / load swapfiles and undo files, or just point to a per-directory workspace?