parse_float support to allow decimal decoding

[vincentmele]

What did you do?

I receive financial market data in JSON format and want to parse it to Decimal, ensuring that there is no loss of fidelity using python's float type.

What did you expect to happen?

The standard json library supports the parse_float parameter in json.loads(). ujson does not appear to have a similar ability.

• OS: Ubuntu 18.04
• Python: 3.7.7
• UltraJSON: 2.0.0-dev4 (but also seen on 1.35)

import json
str_msg = '{"jsonrpc": "2.0","result": {"test1": 935733.817019799357333475932629142801644788893573383506621454627967,"test2": 3383506.81}}'
pprint.pprint( ujson.loads(str_msg))
pprint.pprint( json.loads(str_msg, parse_float=decimal.Decimal))

usjon results in:
{'jsonrpc': '2.0', 'result': {'test1': 935733.8170197994, 'test2': 3383506.81}}

json result in (desired result):
{'jsonrpc': '2.0', 'result': {'test1': Decimal('935733.817019799357333475932629142801644788893573383506621454627967'),'test2':Decimal('3383506.81')}

This may be similar to #124 and #69 but neither relate specifically to decimal.