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'

Gemfile.lock

@@ -205,6 +205,8 @@ GEM
     highline (1.7.10)
     hiredis (0.6.1)
     hkdf (0.3.0)
+    html2text (0.2.1)
+      nokogiri (~> 1.6)
     htmlentities (4.3.4)
     http (3.0.0)
       addressable (~> 2.3)
@@ -601,6 +603,7 @@ DEPENDENCIES
   goldfinger (~> 2.1)
   hamlit-rails (~> 0.2)
   hiredis (~> 0.6)
+  html2text
   htmlentities (~> 4.3)
   http (~> 3.0)
   http_accept_language (~> 2.1)