Support click's auto_envvar_prefix

[nathanakk]

Is your feature request related to a problem? Please describe.
Flask's cli support using FLASK_COMMAND_OPTION environment variables to auto-populate cli arguments/options. (here)
For me, I have to type -d [directory] every time and an option to just use envvar FLASK_DB_RUN_DIRECTORY seems so much better.

Additionally, since -d [directory] is an option for all cli, this can be moved to an option for db group to allow flexibility of flask db -d [directory] [command].

Describe the solution you'd like
Add support for click's auto_envvar_prefix.

[miguelgrinberg]

Could I ask you to try the main branch of the project and confirm that this is working? The variable you need to use for the directory is FLASK_DB_DIRECTORY.