Support new features in Vim 7.3.

Support new features in Vim 7.3.

Enable persistent undo by default if an undofile already exists but not
for temporary files etc.
Show U in the statusline if persistent undo is on.
Show u in the statusline if persistent undo is off but an undofile exists.
Use \u to toggle persistent undo on and off.
Use \U to disable persistent undo and delete the undofile.

Use \CC to toggle ColorColumn at current cursor position.
Use \Cc to remove last set ColorColumn.
Use \Cx to remove all ColorColumns.