Adds tail_before_final_tag and comments features with specs / Updates README

Great library. Thanks for writing this. I pulled it into a project today I'm currently working on, but found I needed a few things that weren't supported by your parser.

1. Adds tail_before_final_tag feature (option set to false by default).
default behavior: `Truncato.truncate("<p><span>some text</span>some more text</p>", max_length: 4) => "<p><span>some...</span></p>"`
new behavior: `Truncato.truncate("<p><span>some text</span>some more text</p>", max_length: 4, tail_before_final_tag: true) => "<p><span>some</span>...</p>"`
(admittedly, this is a very superficial feature, but is something I needed)

2. Adds comments feature (option set to false by default)
Currently Truncato doesn't support html comments. I looked over the Nokogiri::XML::SAX::Parser library and saw there is a comments hook. I added it for completeness.
default behavior: `Truncato.truncate("<!-- some comment --><p>some text</p>") => "<p>some text</p>"`
new behavior: `Truncato.truncate("<!-- some comment --><p>some text</p>", comments: true) => "<!-- some comment --><p>some text</p>"`

3. Correctly handles `<br />` and `<img />` html tags.
Previously, `<br />` and `<img />` tags were not handled correctly (an open and closing tag were applied to the names "br" and "img" 
before: `Truncato.truncate("<br /><p>some text</p>") => "<br></br><p>some text</p>"`
now: `Truncato.truncate("<br /><p>some text</p>") => "<br /><p>some text</p>"`

4. All features covered with specs

5. Corrects a filter spec to show that filtering works as intended

6. Updates README to include both features in the style of the current README

7. Fixes a typo in lib/truncato/truncato.rb, and handles parens around parameters uniformly throughout lib/truncato/truncato_sax_document.rb