Only show marks that exist with autocomplete

This will make it so typing `jump ` and hitting tab will only show marks that are on the system; useful for if you want to create marks for all the projects you edit locally, as well as adding paths to services that tend to run on ubuntu boxes or the such (while keeping dotfiles in sync).

For example, on my MacBook, I don't want to see mysql or redis or asterisk or etc when I type `jump `, and I don't want to see lots of code directories when I type `jump ` on a database server...

`sed -E` should work on both MacOS and most linuxes - it's undocumented on most linuxes, but it's aliased to `sed -r` which does the same thing...