allow to specify at which break token occurance to truncate

[dominik-filip]

Extends :break_token configuration with :break_token_at_count which allows to specify at which occurrence of :break_token to truncate HTML.

Possible use case: truncate first 3 paragraphs.

TruncateHtml.configure do |config|
  config.break_token = '</p>'
  config.break_token_at_count = 3
end