Add `programs.qutebrowser.perDomainSettings` which let's one to set
configuration options for specific URLs [1].
It option doesn't check if the options passed to it are valid, it
translates the config to python code to be written on the file as is.
Mimicking the behaviour of `programs.qutebrowser.settings`.
Added a new test case `test-qutebrowser-url-settings` for testing the
implementation.
[1]: bb7bbb6ead/doc/help/configuring.asciidoc (per-domain-settings)
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
