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.
Merge pull request #39 from Stebalien/presel-modifier

Presel modifier