Add ReverseCoverage::Main.load_results to load multiple result YAML files

[hibariya]

Hello. I added a new method ReverseCoverage::Main.load_results. This allows us to load multiple coverage result YAML files into the @coverage_matrix.
This feature should be useful when running tests in parallel. With this, after each process generates the coverage result YAML file, they can be combined into one and generate a consolidated YAML file, and also, all information will be available in a single HTML file.

usage:

yaml_files = Dir.glob("tmp/reverse_coverage-*.yml").map { File.basename(it) }
ReverseCoverage::Main.load_results file_names: yaml_files

# generates a merged YAML file
ReverseCoverage::Main.save_results

# generates a website with the merged info
coverage_matrix = ReverseCoverage::Main.coverage_matrix
ReverseCoverage::Formatters::HTML::Formatter.new.format(coverage_matrix)