[overlay mode demo] Fixed bug which highlighted the rest of the line

If "{{" was found, the code would highlight the rest of the line
regardless if a matching "}}" or "}}}" was found.
[gfm mode] Added strikethrough implementation
Don't re-highlight lines that were highlighted during drawing
[solarized theme] Remove tab background character

It doesn't belong in a theme.
[vim] Fix bug that vim mode could not be disabled
[vim] simplify selectBlock function
Link to new discussion forum
[vim] improve handling of external selections
[real-world uses] Add three links
Allow calling the constructor with null as place

Issue #2893
[vim mode] Don't directly access .options
[vim] implement gu and gU
Support attach and detach methods for key maps

And adjust vim mode to use these.
Allow non-normalized keys and multi-stroke keys in keymaps

As long as CodeMirror.normalizeKeyMap is called on them before they are
used. This interface is a little weird, but backwards-compatible.
Update the manual to reflect new keymap situation
[meta addon] Added Django and RPM modes
Don't blindly refresh on resize event, compare new size instead
[closebrackets demo] Make mode match editor content
[closebrackets addon] Count quotes as okay to autoclose in front of
Work around Webkit wrapping issue

Issue #2901