[Shell bookmarks] Allow 'warp' command to cd into a subdirectory of a bookmark

[gturi]

Add an optional positional parameter to warp functionality provided by shell-bookmarks init.

After typing a bookmarkName the second parameter will change the target directory to one of its subdirectories.

I.E.

Bookmark systemd points to /etc/systemd.

warp systemd system/multi-user.target.wants

Will cd into /etc/systemd/system/multi-user.target.wants.