feature: added user-configurable options for lint and test sources and test runner

[jonrkarr]

Closes #41

• Please check if the PR fulfills these requirements

• [ ] The commit message is clear and concise
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs/Readme have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Makes lint and test sources configurable by users. This enables other organizational schemes. This also allows pytest to be used rather than unittest discover.

• What is the current behavior? (You can also link to an open issue here) The test and lint sources and test runner are hard coded.

• What is the new behavior (if this is a feature change)? The test and lint sources and test runner are now configurable.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?) No. If the new options aren't set, nx-python behaves the same as before.

• Other information: