Added initial cursor following

This will only highlight up to 100 lines around the current cursor,
making it possible to work in very large files. Compilation will still
be slow, of course. As of now, it refreshes the highlighting whenever
the cursor moves, which is not ideal. It should only happen when
approaching a highlighting boundry.
What is seen is highlighted; updates when needed

50 lines around the cursor are highlighted and only more will be
highlighted if an unhighlighted region enters the window; this greatly
optimizes the viewing of large files.
Updated max visibility default
Added .color_coded file