New features for web parameter parsing module

[namespacestd0]

1. Integrate OpenAPI spec generation with https://github.com/zcqian/biothings.api/blob/openapi/biothings/web/openapi/contexts.py
2. Support forgiving argument processing, for example, for a number limited to 1000, if it's larger than 1000, use 1000 instead of returning an error: https://github.com/biothings/mychem.info/issues/88
3. differentiate argument source, like path argument, body argument, query argument. It will be useful in: https://github.com/biothings/pending.api/issues/20

[namespacestd0]

additional possible features:

1. warn unexpected parameters
2. autosuggest similar names for bad parameters input