[Feature Request] Teach SlateWriter to prepend custom YAML Front Matter

[myabc]

Something along the lines of:

  RspecApiDocumentation.configure do |config|
    config.api_name   = 'My Documentation'
    config.format     = [:html, :slate]
    config.slate_index_front_matter = <<-YAML
toc_footers:
  - <a href='https://github.com/zipmark/rspec_api_documentation/'>Documentation Powered by RSpec API Documentation</a>
includes:
  - custom
  - errors
search: true
YAML
  end

[myabc]

Should make customisation of tabs easier, e.g. #295

[oestrich]

Closing after #375 was merged. If this is still a problem please let me know.