Add support for adding metadata to documents.

Metadata (e.g. title, creation date, tags) can be added to a document
with specially formatted lines at the beginning of a document:

    #+title: The title
    # A headline

Slodown::Formatter#metadata provides access to the extracted metadata:

    (irb)> formatter.metadata[:title]
    "The title"
Document new functionality
Make metadata extraction part of processing pipeline.
Merge pull request #4 from dhamidi/add_support_for_metadata

Add support for metadata