Fix handling of OptionParser return value
Merge pull request #59 from alco/patch-1

Fix handling of OptionParser return value
Refactored content generation from ExDoc into HTMLFormatter
Refactored Retriever such that filtering/nesting is driven by the Formatter
Refactored templating code from HTMLFormatter -> Templates
Cleaned up Retriever tests
Updated tests
Cleaned up comments.
Updated Retriever.docs_from_modules to use just a list of modules.