Added blacklist functionality

[savagedev]

Larry now supports an optional blacklist file containing a list of GitHub repository IDs which get loaded into the cache at runtime and is used to exclude repositories from being published. This implements issue #7.

The blacklist loader uses the existing cache client to provide the interface to the cache store. The loader will always clear out existing blacklist keys before loading the file but will use the key naming convention containing topic and language to keep multiple running Larry processes from interfering with each other.

The blacklist file can contain comments so the repo IDs can be documented with more info if desired.

Changes made:

• The provider now has an isBlacklisted function to check if the repository ID should be excluded.
• The cache functionality has been updated to include a new Scan function which calls a function for every matching key. This is used by the blacklist loader to clear the blacklisted keys before loading the file.
• The config now provides the common cache key prefix logic used by the provider and blacklist loader to determine the proper cache key using the topic and language.
• Readme updated to include the new command line argument and a brief description of the blacklist file format.