Add shellex command

[doxnit]

Adds a new feature - the shellex command. Shellex takes a shell command as an argument, reads its output line by line, and executes those lines as vimb commands.

Thanks @cdlscpmv for idea.

[stale]

This issue has been automatically marked as stale because it has not had activity within the last 60 days.

[fanglingsu]

@doxnit Thank you for the patch and sorry for the late response. For which tasks do you use the :shellex? Yould you please update the manpage under doc/vimb.1.
Could you please reuse existing code of ex_shellcmd. Your ex_shellex looks nearly the same so I assume you could restructure the code so that it can handle :shell and :shellex without replicating the code.