Renamed preface management commands.

OLD:

    preface - edit latex source (pfe)
    preface - generate latex source (pfg)
    preface - interpret latex source (pfi)
    preface - open pdf (pfo)

NEW:

    preface - edit latex source (pe)
    preface - generate latex source (pg)
    preface - interpret latex source (pi)
    preface - open pdf (po)