[MERGE #983] xplat: enhance test/runtests.py to use rlexe.xml
[MERGE #983] xplat: enhance test/runtests.py to use rlexe.xml

Merge pull request #983 from jianchun:pyrl
Use `argparse` module to handle command line arguments. (Now this also
works on Windows -- needs **python 2.7**.)

Read `rlexe.xml` to load test metadata and honor `compile-flags`.

Run all loaded tests and print summary at the end.

Note this is only one step towards current rl test harness. There are lots
of test harness features not implemented yet.

Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore