add result[:mentioned_usernames] for MentionFilter

```ruby
MarkdownPipeline = HTML::Pipeline.new [
  HTML::Pipeline::MarkdownFilter,
  HTML::Pipeline::MentionFilter
]

body = '@fahchen @test'
result = MarkdownPipeline.call(body)
result[:mentioned_usernames] # => ['fahchen', 'test']

no_result_body = 'hello world'
no_result = MarkdownPipeline.call(no_result_body)
no_result[:mentioned_usernames] # => []
```