Add jump plugin, which allows you to easily jump around the file system

Easily jump around the file system by manually adding marks. 
marks are stored as symbolic links in the directory $MARKPATH (default $HOME/.marks)

This functionality complements autojump, which keeps a database of the most-common used directories.

Usage: 

jump FOO: jump to a mark named FOO
mark FOO: create a mark named FOO
unmark FOO: delete a mark
marks: lists all marks