Issue #1358

Adds goGroupLeft/Right, delGroupLeft/Right as alternative to word-based
motion/deletion. Binds ctrl-left/right/del/backspace to these commands.

Also makes word and group-based motion stop at line boundaries.

This reverts commit dd919a2b.

The previous solution broke test doc_docKeepsScroll.

- Do not insert closing bracket if a spacechar or closing bracket
  follows cursor
- Do not handle backspace if selection is not empty
- Do not overwrite anything if selection is not empty
- If selection is not empty and open bracket is inserted - surround
  selection with brackets.

Closes #1351

Closes #1349

Closes #1346

When any specified pair is empty, backspacing will also
delete the closing bracket.

Just a small nit. Without the list check, `getStateAfter`
looks like it will always be true.

Closes #1337

Closes #1336

It had been depending on the list token being highlighted
with the class 'string', which it no longer is in the markdown
mode.

Updated to check the `list` property of the line's state.

These changes are experimental, not documented yet, and might not end up
in the stable API.

Issue #1321

The state.tokenize property could return an array, but the token
method didn't count on that.

Closes #1326