[subset] cmap table dropping

[behdad]

These are very bogus right now, specially the symbol one:

@_add_method(ttLib.getTableClass('cmap')) 
def prune_pre_subset(self, options): 
  if not options.legacy_cmap: 
    # Drop non-Unicode / non-Symbol cmaps 
    self.tables = [t for t in self.tables 
                   if t.platformID == 3 and t.platEncID in [0, 1, 10]] 
  if not options.symbol_cmap: 
    self.tables = [t for t in self.tables 
                   if t.platformID == 3 and t.platEncID in [1, 10]] 

We should change meaning of legacy to mean non-Unicode, but allow both MS and Unicode cmap platform encodings.

Also, add option for "retain one cmap subtable only".