highlight ^old^new pattern

by extending the already in place mechanism of checking for the first
histchar to check for the second as well
Resolves slow performance in brackets highlighter
Chaned main highlighter alorithm to resolve issue #77
Add 'line' highlighter for the whole buffer
zle: don't override yank/yank-pop

`yank-pop` relies on the fact that the last zle command is `yank` or
`yank-pop` to work correctly. Rewriting them prevents this check to work
correctly breaking `yank-pop`.

This fix just disallow overriding of those two zle commands. As a
side-effect, syntax highlighting will not happen when using.

This fixes #99.
Merge pull request #143 from vincentbernat/fix/yank-pop

zle: don't override yank/yank-pop
Merge pull request #142 from jcassee/master

Add 'line' highlighter for the whole buffer
Merge pull request #134 from sonnym/enable_quick_history_substitution_highlighting

highlight ^old^new pattern
Merge pull request #140 from randomize/pr_fix_issue_#77

Pr fix issue #77