Add a --config option to Parser.py, to use a configuration file.

This configuration file can contain a list of transformations to apply to
the test results. This will allow to mark tests as false positives or to
waive failures.

We describe the YAML configuration file format in details in the README.md.
We also add a sample configuration, to demonstrate usage of configuration
files with the already provided sample log and sequence files.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>