title: Configure gatsby-transformer-remark to add default classes
body: <p>I am using gatsby with plugins <code>gatsby-source-filesystem</code> and <code>gatsby-transformer-remark</code> to display markdown files as pages, as described in the <a href="https://www.gatsbyjs.org/docs/adding-markdown-pages/" rel="noreferrer">official docs</a>.</p>
  
  <p>It works great, but I am looking for a way to add default classes to all the elements that are converted from markdown. </p>
  
  <p>Let's say I want each <code>&lt;h1&gt;</code> element to have a class of <code>title</code>, and <code>&lt;h2&gt;</code> elements to have a class of <code>subtitle</code> by default.</p>
  
  <p>I managed to do something like this with <code>gatsby-remark-attr</code>, but with that I can only add classes programmatically in the markdown file. It looks like this:</p>
  
  <pre><code># My markdown heading
  {.title}
  
  ## Subtitle
  {.subtitle}
  </code></pre>
  
  <p>converts to</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;h1 class="title"&gt;My markdown heading&lt;/h1&gt;
  &lt;h2 class="subtitle"&gt;Subtitle&lt;/h2&gt;
  </code></pre>
  
  <p>I am looking for a way to define the default classes once for each element and have them applied automatically, without having to specify them in the markdown files.</p>