Config file format is inconvenient when you have a lot of untyped dependencies

• Are you reporting a bug, or opening a feature request?

Feature request.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

When a project with many dependencies is ported to mypy, it will typically have to tell mypy to ignore the imports of these many dependencies. That means that the mypy.ini config file will look like this

[mypy-numpy]
ignore_missing_imports = True
[mypy-cv2]
ignore_missing_imports = True
[mypy-bson,bson.*]
ignore_missing_imports = True

For big projects, this list will be very long.

• What is the behavior/output you expect?

It would be nice if you could specify the list as a single option in the mypy config file:

[mypy]
ignore_missing_imports =
    numpy
    cv2
    bson
    ...

Begin with the mypy.ini examples in the issue, then locate mypy's configuration parser and tests for per-module options. Check how repeated ignore_missing_imports sections and dotted patterns such as bson.* are currently handled. Done means a single list-style setting applies the option to each named module or pattern.