See http://discuss.codemirror.net/t/possible-bug-in-simple-mode-indenting-behavior/109

Issue #2991

See http://discuss.codemirror.net/t/loadmode-js

Issue #2919

Bumping them for every release just adds too much git
noise and history blowup.

Added `combineTokens` for overlays that can override default
`combine` argument.

It assumed it saw all parts of the stream, and started
doing bad things when that was not the case.

Issue #2410

Issue #2111

Example usage: applying different color schemes to submodes.

Closes #1133