Basic CI and PR policy to ensure integraty of commits

[ddio]

As a project member, I want to make sure the commits I push to Github won't affect existing feature, so to reduce effort for manual code review and communication cost.

This issue is also related to #2 , as we can run unit test also in CI when it's ready.

Acceptance Criteria

1. A CI environment that will run all spiders with simple command, say, minuteago=10 target_cities=台北市 and ensure they get meaningful results.
2. The CI should be triggered on every PR, and everyone must merge code by PR.

[ddio]

Looks like Github built-in CI/CD supports everything we need. Although there seem to be some drawback, they have either been resolved, like live log, or is not a required feature for this project.