Introduce html2text for extracting plaintext from statuses. #236.

Unlike strip_tags, html2text will preserve text present in other nodes,
e.g. anchor tags:

    [1] pry(main)> str = '<a href="http://www.example.com">A link</a>'
    => "<a href=\"http://www.example.com\">A link</a>"
    [2] pry(main)> Html2Text.convert(str)
    => "[A link](http://www.example.com)"
    [3] pry(main)> include ActionView::Helpers::SanitizeHelper
    => Object
    [4] pry(main)> strip_tags(str)
    => "A link"

Preserving the href of an anchor allows keyword mutes to also match on
URLs, which is something that the frontend regex filter can currently
do.

Gemfile

@@ -42,6 +42,7 @@ gem 'fast_blank', '~> 1.0'
 gem 'goldfinger', '~> 2.1'
 gem 'hiredis', '~> 0.6'
 gem 'redis-namespace', '~> 1.5'
+gem 'html2text'
 gem 'htmlentities', '~> 4.3'
 gem 'http', '~> 3.0'
 gem 'http_accept_language', '~> 2.1'