Refactor tests: split into common, implicit and explicit cases

[phha]

By the way, as for splitting the tests into files I would propose to split them into three files:

1. test_configuration_option.py: tests that don't care about the implicit argument (could in principle be run with both values of the argument. if we run them only for one, I would take the default value)
2. test_configuration_option_implicit.py
3. test_configuration_option_explicit.py

I believe most tests are actually in the first category.

Anyhow, this does not necessarily need to be done now and could be made an issue. What remains to be done, though, is to add a sentence on the implicit option in the README.

Originally posted by @ltalirz in https://github.com/phha/click_config_file/pull/7#issuecomment-486169328