Add. Example of basic custom reporter.

[moteus]

For now the only way to pass extra arguments to reporter is a global configuration table. I think LuaCov should documents some restriction for using namespace. E.g. plugin should add only one entry in configuration file and use table if it need more then one option. Or may be we should also create configuration.externals={} for external plagins.

@britzl. I find your luacov fork that generate reports in cobertura format. What do you think about write luacov-cobertura project like luacov-coveralls. May be you have idea about better interface for plugins?

[hishamhm]

Sorry for taking so long to apply this!