Add Extractor class

[noriyotcp]

Separate CLI and logic to extract

Enable to create an instance in irb like this:

❯ irb
> require 'headown'
=> true
> extractor = Headown::Extractor.new('spec/headown/sample.md')
> extractor.headers
=> ["# h1", "## h2", "### h3", "#### h4", "##### h5", "###### h6"]