title: Embed font-awesome icon in Rails button_to
body: <p>I would like to have a button as follow:</p>
  
  <pre><code>[ Sign in with FB]
  </code></pre>
  
  <p>where FB is a font-awesome icon. I tried the following, but couldn't figure out how to embed the icon to the button:</p>
  
  <pre><code>= button_to "Login with", user_omniauth_authorize_path(:facebook)
  </code></pre>
  
  <p>Here is how <code>font-awesome</code> is normally invoked (in haml):</p>
  
  <pre><code>%i.icon-facebook-sign
  </code></pre>
  
  <p>How do I achieve the effect I want?</p>
  
  <p>Thank you.</p>