Add ::createCheckpoint and ::revertToCheckPoint

Signed-off-by: Nathan Sobo <nathan@github.com>
Add ::groupChangesSinceCheckpoint

This groups all changes since the given checkpoint into a single
transaction on the undo history, which will be used to batch together
a set of changes after vim-mode exits insert mode.

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
:pencil: checkpoint methods, “experimental” for now.
Deprecate open-ended transactions
Merge pull request #38 from atom/checkpoints

Add checkpoints as a replacement for open-ended transactions