Feature: Evaluate coverage thresholds against individual files

Add a `check-file-coverage` command with the following behavior:
* Parse coverage JSON files generated by `cover`
* Enforce coverage of statements, branches, lines, and functions
* Enforce coverage on a per-file basis; each file must pass/fail
* Output file name and coverage stats to stdout for failing files
* Call the callback with errors if minimum file coverage not met

NOTE: this command provides the same interface as `check-coverage`

Add test suite for `check-file-coverage` command

Add documentation for `check-file-coverage` command to README