Simplify MWTidy

* Make the internal MWTidy::*clean() functions always return an array of two
  elements: the output buffer and the error buffer.
* Make MWTidy::externalTidy() always read both stdout and stderr. We can read
  stderr after stdout because tidy.c produces output in the same order.
* Remove the $stderr parameter from the private MWTidy::*clean() methods, since
  error output is always returned.
* Merge MWTidy::phpClean and MWTidy::hhvmClean, since the difference between
  them is now small enough that splitting them up is not warranted.
* On HHVM, MWTidy::internalTidy() always returns an empty string for the error
  buffer.

Change-Id: I178b42d6ebdd1a5b9bd5921eb093a6c5014ffa49