config: make redis configuration optional

Default to local redis server.
config: human-readable error

Display a human-readable error in case of configuration error.
config: accept relative paths for certificates

Certificates will be searched relative to the path to the configuration
file.
Merge pull request #2 from vincentbernat/feature/config-enhancements

Configuration enhancement
Merge pull request #1 from vincentbernat/feature/optional-redis-config

config: make redis configuration optional
args: use optparse-applicative to parse arguments

This will be easier to add more arguments later for debugging or stuff
like that.