Add flags to semantic parsing

[kmineshima]

Adding 'flags' to semantic parsing would be useful to generate various formulas from a single set of templates. A template could be written as:

- category: ...
  semantics: ...
  tags: ["tense", "modal"]

and then you could call the script as:

python semparse.py --tags tense modal

in which case, only those templates that have "tense" and "modal" in their tags would be eligible. Also, those templates with no tags would also be eligible.