Feature: add `check-file-coverage` to the command spec.

The existing `check-coverage` command is useful, but it is not able
to enforce code coverage thresholds on a per-file basis. This new
command provides exactly the same interface as the `check-coverage`
command, but enforces the coverage thresholds upon each source file
provided in the coverage JSON reports.

In addition to throwing an exception if the coverage requirements
are not met, `check-file-coverage` logs the name and coverage
statistics for each failing file to standard output.

This commit includes:
* Documentation of command
* Command implementation
* Test cases for command