exec demo

Running <!exec git log --oneline -10> in local repo clone:

87fb724 Avoid infinite recursion after <!else> token with new line
62128c8 Fix <!method> jump to the correct following <!method>
8c94875 Add '_' and '-' for !htex tag name (e.g. <!include-markdown>)
e0e01d5 Add handling for future <!if> elements (#2)
a2120bc [ci] Update actions
380fd4b Add TokensIter
d97e504 Add suppor to use operators like <!a=b> without spaces
6c49148 [ci] Add all platforms for testing
91cf426 Fix layout tests on Windows
5dc939a Fix elements that receive a path (e.g. <!layout>/!include>/etc.)

<!layout /.includes/layout.htex>
<article>
  <h2>exec demo</h2>
  <p>Running <code>&lt;!exec git log --oneline -10&gt;</code>
     in <a href="https://github.com/dacap/htex/commits/main/">local repo clone</a>:</p>
  <pre><code><!exec git log --oneline -10></code></pre>
</article>
<article>
  <code>exec.htex</code> source file:
  <pre><code class="language-html"><!include-escaped exec.htex></code></pre>
</article>