title: Package `sentimentr`: how remove emoticons and stopwords before `sentiment_by`
body: <p>Here is a basic sentiment example. The text data is splitted into sentences via the <code>get_sentences</code> function. With <code>sentiment_by</code> we approximate the sentiment (polarity) of text for an entire element of a list (<code>mytext</code> in this example).</p>
  
  <p>E.g. for the example:</p>
  
  <pre><code>library(sentimentr)
  
  mytext &lt;- c(
    'do you like it?  But I hate really bad dogs',
    'I am the best friend.',
    'Do you really like it?  I\'m not a fan'
  )
  
  mytext &lt;- get_sentences(mytext)
  sentiment_by(mytext)
  </code></pre>
  
  <p>I obtained the following result:</p>
  
  <pre><code>   element_id word_count       sd ave_sentiment
  1:          1         10 1.497465    -0.8088680
  2:          2          5       NA     0.5813777
  3:          3          9 0.284605     0.2196345
  </code></pre>
  
  <p>Before applying sentiment function, I would like to remove stop words, number, emoticons from <code>mytext</code>. I figured I could use, e.g:</p>
  
  <pre><code>library("tm")
  tm_map(mytext, removeNumbers)
  tm_map(mytext, removeWords, stopwords())
  </code></pre>
  
  <p>but I obtained:</p>
  
  <pre><code>Error in UseMethod("tm_map", x) : 
   no applicable method for 'tm_map' applied to an object of class "c('get_sentences', 
  'get_sentences_character', 'list')"
  </code></pre>