launcher: option to make command line args take precedence (#327)

## Link to GitHub Issue, if one exists
Fixes #190 

## Description of change
When `-cmdoverride` is specified, any command line arguments are taken
first before merging with what is in the config file.

## Testing
Still testing...

src/spice2x/cfg/option.h

@@ -43,6 +43,7 @@ public:
     std::string value;
     std::vector<Option> alternatives;
     bool disabled = false;
+    bool conflicting = false;
 
     explicit Option(OptionDefinition definition, std::string value = "") :
         definition(std::move(definition)), value(std::move(value)) {