Add. Example of basic custom reporter.

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](https://github.com/britzl/luacov) fork that generate reports in cobertura format.
What do you think about write `luacov-cobertura` project like [luacov-coveralls](https://github.com/moteus/luacov-coveralls).
May be you have idea about better interface for plugins?