Merge pull request #38 from Stebalien/fix-segfaults

Don't segfault.
Don't skip matching monitors
Use '--' as first argument for the `set` built-in

That's what dash's manual specifies.
Add mode modifier

Allows selecting windows based on their mode (manual/automatic).  This
is useful for duplicating the old shift-can-be-transplant behavior.

    # Usage: squish DIR
    squish() {
        dir=$1
        bspc window -w ${dir}.manual || bspc window -s ${dir}
    }
Document mode modifier.