Support repeated instances of flags

[trevorld]

• Currently {getopt} returns a named list which although convenient for many use cases is an awkward data structure if we care about repeated instances of a flag

• In comparison for example the Python getopt module instead

  The options occur in the list in the same order in which they were found, thus allowing multiple occurrences. Long and short options may be mixed.

• In particular this would be convenient for supporting Adding support for actions r-optparse#22 and allow repeated instances of same flag/option on command line r-optparse#36