TCD plugin: Tab-local Current working directory

This plugin provides the TCD command and SetTCD function. These two will
run :lcd for all windows in the current tab, thus making the whole tab
use a different working directory.

Opening new windows and tabs is no longer a problem as the "lcd" are
inherited by the new windows.
Set TCD when file is read in a fresh tab.

No need for any terrible hacks as Vim's native ":lcd" support will
change the current working directory for us.