feat: add support for single-quoted doc

[zephvr]

This PR fulfill #84 request to support single-quoted doc

With it

# code.py
from pathlib import Path
from tap import Tap

class Args(Tap):
    db: Path
    "database file"

Args().parse_args()

gives

$ python code.py -h
usage: code.py --db DB [-h]

options:
  --db DB     (Path, required) database file
  -h, --help  show this help message and exit

Of course tell me if I need to change my MR, I'm still learning my way to contribute to open source project I like