Add option to exclude files from testing

[midokura-silvia]

As described in #1, there is currently no way to exclude files from testing.
This pull request implements a simple way of excluding files from the command line via substring matching.

[amit1rrr]

@midokura-silvia Thank you for your contribution. I really like this simple approach to exclude files. I have made some suggestions to make it more robust. If we can work through those I'd be happy to merge it.

Whenever ready, please add some tests to make sure everything is working as intended and future regressions are caught beforehand. You can refer to the existing tests - https://github.com/ReviewNB/treon/tree/master/tests

[amit1rrr]

Thank you for making the changes & putting in the tests. I tested with your branch & works as expected. I just have a few nitpicks after which we can merge.

[amit1rrr]

There is one small linting error that is why travis ci is failing. You can run make lint to see it. Once fixed good to merge.

[amit1rrr]

Merged!

@midokura-silvia Thank you for your contribution!

[amit1rrr]

@midokura-silvia Released this change in 0.1.3 in case you want to start using this option right away.

https://pypi.org/project/treon/