title: Assert string only contains letters
body: <p>I have seen methods like <code>isAlpha()</code>, but it accepts spaces and punctuations, which I don't want. Any way to check that a string contains only big or upper case alphabet letters?</p>
  
  <p>E.g. psudo:</p>
  
  <pre><code>"asdf".isLetters() -&gt; true
  
  "as df".isLetters() -&gt; false
  "as. df:".isLetters() -&gt; false
  </code></pre>