Preferred mechanism for command line overrides

[KiranEiden]

Currently the command line options --tolerance and --check_performance (addressing issue #5 and issue #15) will override any values set in the configuration files if supplied. Would the option of a "soft" override, where those settings (tolerance and performance threshold/runs to average) are only overridden for individual tests which do not have a tolerance setting or do not already check performance, be a useful addition?

If so, what would be the preferred implementation? One option is to introduce additional command line flags; another is to treat a negative number for a given setting as a soft override for that field.