specify arguments in usage text
add --remove-files-in
create
add --keepstamps feature

parsing the "From " date was only ever used to override the Date
header? (To be fed to optional perl code, or what?)
carry over timestamps from Maildir (using localtime)

Need to adapt cj-mbox2maildir as well if want to change for gmtime,
but Thunderbird actually uses localtime and I think yes, I've seen
that used before. Chj::Parse::Mbox assumes the same.