Ability to ignore set of notebooks from test suite

[amit1rrr]

Usually we have some rough notebooks and some final notebooks in the same repo. We never want to run tests for rough notebooks. It would be great to have .ci_ignore file that can be used to specify list of notebooks/directories that should be excluded from test suite.

Current workaround is to actually specify path of the notebooks/directories that needs to be included in the test suite.

[NiNXHeart]

Enhancement Will functon better

[amit1rrr]

Thanks to the contribution from @midokura-silvia we now have ability to exclude notebooks & directories from testing.