Add scd plugin for smart change of directory.

I would like to contribute a new plugin scd for smart change of directory.
scd is somewhat similar to an independently developed *z* plugin, however
scd offers a selection menu in case of multiple matches, uses a different
formula for directory ranking, time-stamped index that is appended rather
than rewritten and can be also used from Vim editor or IPython shell.

The added commit is a copy of the scd-tracker branch pavoljuhas/oh-my-zsh@9d04d8ca78030d4c9af4bf0f58b49532a6ef1b35.