title: How to markdown/html tables containing plus/minus
body: <p>I use a chain of </p>
  
  <pre><code>knitr::knit2html("test.Rmd") # generates test.md &amp; test.html
  rmarkdown::render("test.md") # overwrites test.html
  </code></pre>
  
  <p>to generate an html report.</p>
  
  <p>This chain provides good functionality as my report usually combines pictures, tables &amp; text. If I ran only</p>
  
  <pre><code>knitr::knit2html("test.Rmd")
  </code></pre>
  
  <p>"test.html" will be generated, but it looks awkward, i.e. pictures not shown correctly. </p>
  
  <p>Normally, this works fine, but this time sample names that are headers of a table contain '+' or '-'. </p>
  
  <pre><code>|               | IP_gene8-_1st| IP_gene8+_1st| 
  |:--------------|-------------:|-------------:|
  |IP_gene8-_1st  |     1.0000000|     0.4357325|
  |IP_gene8+_1st  |     0.4357325|     1.0000000|
  </code></pre>
  
  <p>"test.html" generated by <code>knit2html("test.Rmd")</code> will contain a valid table, but other pictures are not shown correctly. </p>
  
  <pre><code>&lt;table&gt;&lt;thead&gt;
  &lt;tr&gt;
  &lt;th align="left"&gt;&lt;/th&gt;
  &lt;th align="right"&gt;IP_Rad18-_1st&lt;/th&gt;
  &lt;th align="right"&gt;IP_Rad18_1st&lt;/th&gt;
  &lt;/tr&gt;
  &lt;/thead&gt;&lt;tbody&gt;
  &lt;tr&gt;
  &lt;td align="left"&gt;IP_Rad18_1st&lt;/td&gt;
  &lt;td align="right"&gt;1.0000000&lt;/td&gt;
  &lt;td align="right"&gt;0.4357325&lt;/td&gt;
  &lt;/tr&gt;
  &lt;tr&gt;
  &lt;td align="left"&gt;IP_Rad18_1st&lt;/td&gt;
  &lt;td align="right"&gt;0.4357325&lt;/td&gt;
  &lt;td align="right"&gt;1.0000000&lt;/td&gt;
  &lt;/tr&gt;
  &lt;/tbody&gt;&lt;/table&gt;
  </code></pre>
  
  <p>Running <code>rmarkdown::render("test.md")</code> produces a "test.html" with a table as text only, but e.g. pictures shown correctly. The crappy table output looks like this:</p>
  
  <p>|              | IP_gene8-_1st| IP_gene8+_1st| 
  |:-------------|-------------:|-------------:|
  |IP_Rad18_1st  |     1.0000000|     0.4357325|
  |IP_Rad18_1st  |     0.4357325|     1.0000000| </p>
  
  <p>Usually, '+' and '-' can be protected using '/', but this does not have any effect in the table context. </p>
  
  <p>Is there any way to trick <code>rmarkdown::render()</code> to create a valid html-table?</p>