feat: add Enum support with cleaner help message

[zephvr]

Hi
I would like to propose this PR to fulfill #70 request

With it this code

# code.py
from enum import Enum
from tap import Tap

class MyEnum(str, Enum):
    A = "aaa"
    B = "bbb"
    C = "ccc"

class MyArgs(Tap):
    myEnum1: MyEnum = MyEnum.A  # My comment

print(MyArgs().parse_args())

Gives

$ python code.py -h
usage: demo.py [--myEnum1 {aaa,bbb,ccc}] [-h]

options:
  --myEnum1 {aaa,bbb,ccc}
                        (default=aaa) My comment
  -h, --help            show this help message and exit

Tell me if there is anything that needs a change before merging 😃