Confusing file exclusion behavior should be documented

* **Version**: 7.1.0
* **Platform**: Linux

`test-exclude` unilaterally excludes files in parent (relative to cwd) directories. This was very confusing when I was trying to generate coverage reports from within my test directory and the tests were only referencing files in parent directories. The generated reports would always show no results no matter what options I passed to `c8` (e.g. `--all`, `--include`, `--exclude`, etc.).

At the very least this should be documented in the readme and additionally it would be nice to have the behavior be overridable (this would require changes to `test-exclude` since as of this writing it does not support overriding its relative path check).