Resolve ours-then-theirs and theirs-then-ours correctly during rebases

The `acceptOursThenTheirs` and `acceptTheirsThenOurs` methods in `ConflictMarker` assume the merge order for `Sides` within the document: ours, navigator, theirs. Because "ours" and "theirs" are flipped during a rebase, the resolve methods were manipulating the wrong markers and botching everything up.

I'm adding a `followingMarker` property to each `Side` during the conflict marking process to remember the marker that should be extended if the side's text is manipulated programmatically.

This will fix #45.