Allow passing extension parameters via CLI

[GuySartorelli]

PHPUnit XML configuration provides a parameter element for passing parameters to an extension. This can be very useful, but currently means we have to modify the configuration file if we want to change parameter values for a given test execution run.

There is an undocumented --extension option to pass an extension class via CLI - but no way to currently pass extension parameters.

I assume this is because it would be difficult with a single flag like --extension-parameter to declare which extension class the parameter is for, but for cases where there is only one extension class a flag like that would be very useful.

Perhaps it could just throw an exception or warning if there are multiple extension classes, or only apply to the first extension?