called before the addon
changes status to "Enabled". By returning a non-nil <code>error</code> value, the addon
will not become enabled.</p>
<div class="codehilite"><pre><span></span><span class="kd">func</span> <span class="p">(</span><span class="nx">p</span> <span class="o">*</span><span class="nx">Post</span><span class="p">)</span> <span class="nx">BeforeEnable</span><span class="p">(</span><span class="nx">http</span><span class="p">.</span><span class="nx">ResponseWriter</span><span class="p">,</span