Added concept of "letter"

This makes (letter --> word) work in the same way as (digit --> integer).

Although the constructed regex will now be slightly different for anything containing words, this is fully backwards compatible.

lib/verbal_expressions.rb

@@ -71,9 +71,14 @@ def tab
     add('\t')
   end
 
-  # Any alphanumeric
+  # Any single alphanumeric
+  def letter
+    add('\w')
+  end
+
+  # Any word (multiple alphanumerics)
   def word
-    add('\w+')
+    one_or_more { letter }
   end
 
   # Any single digit