Optional skip_schemas

[stain]

Fixes common-workflow-language/cwltool#433

I would ideally also then want to remove "Did you include a $schemas section" from validate.py - but we don't know if the user want to do a strict vs non-strict.

[mr-c]

(env) mcrusoe@mrcdev:~/common-workflow-language$ ./run_test.sh RUNNER=cwltool EXTRA=--skip-schemas
--- Running conformance test v1.0 on /home/mcrusoe/cwltool/env/bin/cwltool ---
/home/mcrusoe/cwltool/env/bin/cwltool 1.0.20170623150402
Test [87/87] All tests passed

All tool tests succeeded

I am pleasantly surprised that this didn't fail any tests!

[MarkRobbo]

@mr-c Double edged sword though! Maybe that indicates that some tests should be there for add_schemas 😄