Inspect meta of `appendedTransaction` for possible `addToHistory:false`

[romansp]

Resolves #141.

Using the same approach as in native prosemirror-history, added inspection of appendedTransaction meta for possible addToHistory: false to prevent root transaction from appearing on the undo/redo stack.

Added a test case that covers expected behavior.

[BrianHung]

Great fix for developers trying to chain multiple commands together but only have one undo / redo for all using the appendedTransaction meta.