Checkenv

* Added ability for `check()` to throw exceptions instead of killing the running process with `raise_exception=True`
* Added ability to silence all output to `stdout` with `no_output=True`
* Updated documentation with the `filename` parameter feature that allows you to specify an input JSON file with a different name than `env.json`
* Updated README.md with usage instructions for these new features
* Increased code coverage to 95%+

* Expanded supported Python interpreter versions - `checkenv` now supports Python versions 2.7, 3.5, 3.6, 3.7, and 3.8.
* Refactored the code with classes - although this does not add additional functionality, the code is cleaner, easier to understand, and better documented for future improvements
* Added tests with `pytest` and `tox`
* Every pushed branch undergoes automated testing with CircleCI
* Started tracking code coverage percentage and currently hovering around ~50% (to be improved)

Initial release with decent feature parity compared to npm's `checkenv`