Implement multicolumn support for slide formats.

The structure expected is:

    <div class="columns">
      <div class="column" width="40%">
        contents...
      </div>
      <div class="column" width="60%">
        contents...
      </div>
    </div>

Support has been added for beamer and all HTML slide formats.

Closes #1710.

Note:  later we could add a more elegant way to create
this structure in Markdown than to use raw HTML div elements.
This would come for free with a "native div syntax" (#168).
Or we could devise something specific to slides

1 files changed, 91 insertions, 0 deletions

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+```
+% pandoc -t revealjs
+# Slide one
+
+<div class="columns">
+<div class="column" width="40%">
+- a
+- b
+</div>
+<div class="column" width="40%">
+- c
+- d
+</div>
+<div class="column" width="10%">
+ok
+</div>
+</div>
+^D
+<section id="slide-one" class="slide level1">
+<h1>Slide one</h1>
+<div class="columns">
+<div class="column" width="40%" style="width:40%;min-width:40%;vertical-align:top;">
+<ul>
+<li>a</li>
+<li>b</li>
+</ul>
+</div>
+<div class="column" width="40%" style="width:40%;min-width:40%;vertical-align:top;">
+<ul>
+<li>c</li>
+<li>d</li>
+</ul>
+</div>
+<div class="column" width="10%" style="width:10%;min-width:10%;vertical-align:top;">
+<p>ok</p>
+</div>
+</div>
+</section>
+```
+
+```
+% pandoc -t beamer
+# Slide one
+
+<div class="columns">
+<div class="column" width="40%">
+- a
+- b
+</div>
+<div class="column" width="40%">
+- c
+- d
+</div>
+<div class="column" width="10%">
+ok
+</div>
+</div>
+^D
+\begin{frame}{%
+\protect\hypertarget{slide-one}{%
+Slide one}}
+
+\begin{columns}[T]
+\begin{column}{0.40\textwidth}
+\begin{itemize}
+\tightlist
+\item
+  a
+\item
+  b
+\end{itemize}
+\end{column}
+
+\begin{column}{0.40\textwidth}
+\begin{itemize}
+\tightlist
+\item
+  c
+\item
+  d
+\end{itemize}
+\end{column}
+
+\begin{column}{0.10\textwidth}
+ok
+\end{column}
+\end{columns}
+
+\end{frame}
+```
+