add configuration option check

This adds a new function girara_config_check() to girara which can be used to check the presents of a specified option and corresponding value inside the configuration files.

Unlike girara_config_parse(), which directly saves the parsed configuration values into a girara session, this new function can be used before girara session or any UI elements have been initialized.

This allows the verification of a configuration setting before girara_config_parse() can be used.

Related bug: zathura#276