Chore/56 option to include large unused dependencies

[saattrupdan]

Changed

• Changed default embedder in RagSystem to OpenAIEmbedder from E5Embedder.

Added

• Added new e5 and cpu extras, where e5 installs the sentence-transformers
  dependency required for the E5Embedder, and you can add cpu to install the
  CPU-version of torch to save disk space (note that this is not available on MacOS,
  however).
• Added new from_config class methods to RagSystem and Demo to create instances
  from a configuration file (YAML or JSON). See the readme for more information.
• Added new ragger-demo and ragger-compile command line interfaces to run the demo
  and compile the RAG system, respectively. Compilation is useful in cases where you
  want to ensure that all components have everything downloaded and installed before
  use. Both of these take a single --config-file argument to specify a configuration
  file. See the readme for more information.

This closes #56.