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)
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