Add command line option for producing TAP output.

[siffiejoe]

The -t flag is still free. Using TAP would allow it to include testing of Lua modules in a larger test suite that might use different programming languages. One additional effect of the TAP mode would be the change of the output file handle from io.stderr to io.stdout.

Because the number of test cases is unknown beforehand, the "plan" would be omitted (it is optional) or added at the very end. Support for # TODO or # SKIP would be manual (meaning, if you use those strings in the assert messages, they will show up in the output).

[siffiejoe]

This is now implemented in master.