Improved parenthesis highlighting in vi insert mode

[tomaneo]

Matching parenthesis highlighting was not very useful in vi insert mode, because it didn't highlight the opening parenthesis for just inserted closing parenthesis. Only after switching back to normal mode you could see the matching pair highlighted.

This patch changes insert mode behaviour to use parenthesis before the cursor as the highlighted pair in insert mode. This way it is also similar to emacs mode.

Before the patch:

After the patch: