adding details pulldown to selected lessons

[mistrm82]

This is an HTML tag taht would be useful for "try it on your own" sections, where the code is hidden initially but clicking on it will make it available

```bash $ idr --samples Pou5f1-rep1_sorted_peaks.narrowPeak Pou5f1-rep2_sorted_peaks.narrowPeak \ --input-file-type narrowPeak \ --rank p.value \ --output-file Pou5f1-idr \ --plot \ --log-output-file pou5f1.idr.log ```

[rkhetani]

<details>
<pre>
<code>
## plot using ggplot2

ggplot(melted_top20_sigOE) +
geom_point(aes(x = gene, y = normalized_counts, color = sampletype)) +
scale_y_log10() +
xlab("Genes") +
ylab("Normalized Counts") +
ggtitle("Top 20 Significant DE Genes") +
theme_bw() +
theme(axis.text.x = element_text(angle = 45, hjust = 1)) +
theme(plot.title=element_text(hjust=0.5))
</code>
</pre>
</details>

[rkhetani]

The method listed above works nicely for html rendered markdown, looks a little funky without rendering.

[mistrm82]

Does anyone want to take a stab at this? It would be nice feature to have