Issue #3862

`serializeable` → `serializable`
Replace `an` with `a` 
Remove extra `command`

This patch enables styling of active line numbers by
adding a `CodeMirror-activeline-gutter` class to the
line's gutter space.

Issue #3790

Issue #3736

Closes #3742
Closes #3744

Issue #3730

Issue #3722

Issue #3723

It took me forever to figure out why vim mode wasn't working, and all because the manual said to set the "keymap" option and not the "keyMap" option. I also added quotes to make it clear that "vim" is a string.

Issue #3682