Markdown conversion stream -> stream

[serj-lotutovici]

Currently the MarkdownProcessor takes in and returns a String. This can become very slow if the markdown if very large. Usually the actual markdown is accessed throw some io operation, like reading from a file of an http call, which with the current implementation requires the caller to convert the raw stream into an String. It would be much more efficient if the MarkdownProcessor would read the markdown dirrectly form the InputStream, and allow a String as a separate option.