Add cobertura reporter as a default for use with Jenkins

I want to integrate the coverage stats with some Jenkins jobs. To do this I need to output the coverage stats in cobertura format. Instanbul supports this format, however this module did not have a mechanism for specifying new reporters.

For the moment I have just added a new reporter line into #writeReportsInternal, not sure if this is the best way or you would want a better way of specifying reporters.