tpl/math: Add log function

It might be very useful for building tag clouds.

content/templates/functions.md

@@ -432,6 +432,12 @@ favicon.ico: {{.Width}} x {{.Height}}
 <td><code>{{div 6 3}}</code> → 2</td>
 </tr>
 
+<tr>
+<td><code>math.Log</code></td>
+<td>Natural logarithm of one float.</td>
+<td><code>{{math.Log 1.0}}</code> → 0</td>
+</tr>
+
 <tr>
 <td><code>mod</code></td>
 <td>Modulus of two integers.</td>