pairtaghighliter: Fix crash on plugin unload

Fix crash on plugin unload if the plugin didn't run on any editor, or
if the last editor it ran on became invalid.

The global 'sci' variable wasn't set in 'plugin_cleanup()' and thus may
either have been uninitialized or initialized to a value not valid
anymore.

The simplest way to reproduce the crash was to load and then
immediately unload the plugin, without leaving the plugin manager
dialog.
pairtaghighliter: Remove a global variable

Remove use of the 'sci' global variable and instead pass it as an
argument to each function requiring this data.

The use of global variables makes code harder to understand and debug,
and often leads to unexpected issues.  Here, it lead to a crash when
unloading the the plugin because an apparently innocent call used this
global variable before its initialization.  This was fixed in previous
commit, but removing use of the global variable hopefully will prevent
such bugs from reappearing later.
Merge pull request #100 from b4n/fixes/pair-tag-highlighter

Fix crash on unloading plugin PairTagHighlighter