Allow blocks to parse/to_html for custom behaviour

[rgarner]

The default Vandamme::Parser#parse behaviour only allows the output of
hashes containing one key – either version or release date – via use of
the :match_group option to Vandamme::Parser#initialize. This makes it hard
to get a hash with all of: versions, date, and content.

Without breaking the current interface requiring a major version change,
allow custom behaviour to both #parse and #to_html by allowing blocks.

For #parse, yield the changelog hash, the header match, and the current
content. This means

parser.parse do |result, header_match, content|
  result[header_match[0]] = { date: header_match[1], content: content }
end

can produce a hash such as:

{
  '1.0.0' => { date: '2017-09-19', content: "* My\n* Changes" }
}

and a corresponding

parser.to_html { |hash| hash[:content] }

can generate its html.