Mocha.prototype.reporter is now Mocha.prototype.reporters and can take the same types of arguments
as before, plus a list of reporter names as a comma separated string, or a list of constructors.
This allows using more than one reporter.
Replaced command line switch --reporter <name> by --reporters <name>,... so that users
can pass a comma separated list of reporters on the command line.
The old --reporters command line switch used to list available reporters is now --list-reporters.