[Feature] Support consistent undo behavior

[terryma]

Currently the plugin's undo behavior is unpredictable, both inside and outside of multicursor mode. For example, text inserted into the buffer during multicursor mode is undone one character at a time in all virtual cursor locations. The correct behavior should be consistent with Vim - everything entered during insert mode before exiting back to normal mode is 1 undo block.

Implementing this might require some clever hackery with using undojoin, further investigation is needed.

[d11wtq]

I can confirm that this now behaves much more sanely. I did just manage to get it to do something odd, but I can't reproduce it now, as I'm not sure what I did. It undid everything I inserted as a single batch, but the file-modified status was showing 59 changes. Pressing u 59 times made Vim agree there were no changes, but nothing was actually being changed at all. I can live with that though... I was mashing keys right before it trying to make #28 break (happy to say unsuccessfully).