Configure the scope according to the CLI options on re-evaluation of Guardfile

Hello,


I was trying to focus on one of the plugins specified in my `Guardfile` and, hence, provided the `-P` option to the CLI.  When I started adjusting `Guardfile` later on, Guard was re-evaluating it as expected, but it wasn’t taking into account the CLI options.  I would appreciate any feedback. If it is the desired behavior, please do not hesitate to close the pull request.


Best wishes,
Ivan
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