py37 dataclass support

[cleoold]

Thanks for your awesome library.

This can help with #46.

Behavioural difference from that asdict() solution: asdict() will try to convert nested dataclasses to dicts as well (creating many copies), and producing different outputs:

>>> from dataclasses import dataclass, asdict
>>> from tabulate import tabulate

>>> @dataclass
... class Mydata:
...     @dataclass
...     class Yolo:
...         x: int
...         y: int
...     name: str
...     yolo: Yolo

>>> d = [Mydata('origin', Mydata.Yolo(0, 0)), Mydata('v', Mydata.Yolo(22, -18))]
>>> print(tabulate(map(asdict, d), headers='keys'))
name    yolo
------  -------------------
origin  {'x': 0, 'y': 0}
v       {'x': 22, 'y': -18}

>>> print(tabulate(d, headers='keys'))
name    yolo
------  ------------------------
origin  Mydata.Yolo(x=0, y=0)
v       Mydata.Yolo(x=22, y=-18)

The latter is more accurate.