Make jedai-core Extensible

[allan-shoup]

Users of jedai-core are unable to extend the library to utilize a custom similarity metric or entity matching method due to the enums defined in the project (e.g. SimilarityMetric, EntityMatchingMethod, BlockCleaningMethod, etc.). Instead, if these features utilized an extension mechanism (for example, java.util.ServiceLoader or something equivalent), custom features would be possible.