Provide an api for other plugins to suspend autosave if they have a edit panel open

[c10b10]

I've run into the same issue (which I thought to be the fault of the packages themselves) caused by autosave in the following packages:

• https://github.com/sQu1rr/nerd-treeview
• https://github.com/emmetio/emmet-atom

Each time an atom panel is launched by a package (NT launches a modal when you want to create a new file, and Emmet creates a panel in interactive mode) and atom is unfocused, autosave kicks in, and wrecks havoc (the modal disappears in NT, the emmet preview changes are saved which results in the need to undo if one unfocused to quickly check syntax).

If autosave could provide a mechanism for temporary disabling, plugins could make use of it when they have pending actions that would be ruined by autosaving.