Chain undos together in insert mode. Prevent ghostly cursors.

This commit provides basic chaining of undos in insert mode. Previously, undos
of changes made in multicursor mode are done one character at a time. This
change should chain all inserts into a single undo block, and undo them
together. This fixes #22.
We're also adding support to detect non-terminating single key commands. For
example, previously, 'd' would break multicursor mode and cause these ghostly
cursors to linger on screen, where the only way to get rid of them is to press
`<C-n>` somewhere to start a new multicursor mode. Now, these unterminated
commands are detected and simply cause a nop, execution continues and we're left
in multicursor mode. This paves the way to support multikey commands in the
future. This fixes #28.