[WIP] Update allowed tags

[westonruter]

See #804 for details. This doesn't yet result in successful generation of the allowed tags. Currently fails with:

python3 amp_wp_build.py
[[amp_wp_build.py SetupOutDir]] - entering ...
[[amp_wp_build.py SetupOutDir]] - ... done
[[amp_wp_build.py GenValidatorProtoascii]] - entering ...
[[amp_wp_build.py GenValidatorProtoascii]] - ... done
[[amp_wp_build.py GenValidatorPb2Py]] - entering ...
[[amp_wp_build.py GenValidatorPb2Py]] - ... done
[[amp_wp_build.py GenValidatorProtoascii]] - entering ...
[[amp_wp_build.py GenValidatorProtoascii]] - ... done
[[amp_wp_build.py GeneratePHP]] - entering ...
[[amp_wp_build.py ParseRules]] - entering ...
Traceback (most recent call last):
  File "amp_wp_build.py", line 604, in <module>
    Main()
  File "amp_wp_build.py", line 601, in Main
    GeneratePHP(out_dir)
  File "amp_wp_build.py", line 118, in GeneratePHP
    allowed_tags, attr_lists, versions = ParseRules(out_dir)
  File "amp_wp_build.py", line 359, in ParseRules
    text_format.Merge(open(specfile).read(), rules)
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 525, in Merge
    descriptor_pool=descriptor_pool)
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 579, in MergeLines
    return parser.MergeLines(lines, message)
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 612, in MergeLines
    self._ParseOrMerge(lines, message)
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 627, in _ParseOrMerge
    self._MergeField(tokenizer, message)
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 727, in _MergeField
    merger(tokenizer, message, field)
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 815, in _MergeMessageField
    self._MergeField(tokenizer, sub_message)
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 727, in _MergeField
    merger(tokenizer, message, field)
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 866, in _MergeScalarField
    value = tokenizer.ConsumeString()
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 1229, in ConsumeString
    the_bytes = self.ConsumeByteString()
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 1244, in ConsumeByteString
    the_list = [self._ConsumeSingleByteString()]
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_format.py", line 1269, in _ConsumeSingleByteString
    result = text_encoding.CUnescape(text[1:-1])
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_encoding.py", line 103, in CUnescape
    result = ''.join(_cescape_highbit_to_str[ord(c)] for c in result)
  File "/usr/local/lib/python3.6/site-packages/google/protobuf/text_encoding.py", line 103, in <genexpr>
    result = ''.join(_cescape_highbit_to_str[ord(c)] for c in result)
IndexError: list index out of range

Fixes #804 (and potentially many other issues)