Simplified OAuth2 flow

[burnash]

This is a first attempt to implement a simplified authentication for gspread. Inspired by excellent gspread-pandas I'd like to cut down the boilerplate for constructing credentials, so you can get the authorized client instance in your code with just:

import gspread
gc = gspread.oauth()

On the first run, this will look for OAuth 2.0 Client credentials in ~/.config/gspread/credentials.json, open a browser window with the application authorization request, get the user's consent and store authorized credentials in the config directory. Next runs will use stored credentials.

Open questions:

1. Credentials security.
2. Default scope.
3. With this added convenience, should client credentials replace service accounts as recommended flow in the docs. This is related to (1) and (2) since a service account does not have access to any spreadsheet unless a spreadsheet is explicitly shared with this account. Using OAuth 2.0 Client credentials, on the other hand, gives the script direct access to the user data.

[burnash]

Ok, let's merge it and try a soft launch :)