add sandbox command line option

Open valoq requested to merge valoq/zathura:master into develop

This commit adds an alternative command line option to set the sandbox mode as an alternative to the configuration option.

One example use cases would be to have the sandbox set to strict in the configuration for general use and use the command line option --sandbox=normal (or none) when using zathura in something like vimtex where we want all features and the file is safe anyway.

The command line option is prioritized if there is a different setting in zathurarc

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