* Add test support for multi-file output.
* Add -update support for multi-file output tests.
* Add support for string output in multi-file output mode.
* Rename 'stringOutput' to 'stringOutputMode' to better express what it does
* Refactor main_test to make it less nested.
This also causes the -update flag to output a list of files which
have been updated. This does not include the paths which are deleted
for multi-file tests.
