Add a feature to optionally direct the test output to a file.

When using mocha from within a framework like grunt, it can be useful to direct the test output to a file. For example, when using the xunit reporter, then all of the output from various test suites could be collected and parsed after the fact.

The implementation creates a new output library and changes all of the test reporters to call output.log or output.write instead of directly calling console.log or process.stdout.write.

It also adds a command line option "-O <file>" to enable the redirection.

Also added a test rule to the Makefile that lists all of the reporters and verifies that the output is properly generated when the option is enabled.