Pa11y testing results
```
> pa11y-ci:sitemap
> pa11y-ci --sitemap http://localhost:4000/semester-biology/sitemap.xml --sitemap-exclude "/*.pdf|Knitr-BBS-R.html|paper.*"
The sitemap "http://localhost:4000/semester-biology/sitemap.xml" could not be loaded
```
Pa11y testing results
```
> pa11y-ci:sitemap
> pa11y-ci --sitemap http://localhost:4000/semester-biology/sitemap.xml --sitemap-exclude "/*.pdf|Knitr-BBS-R.html|paper.*"
The sitemap "http://localhost:4000/semester-biology/sitemap.xml" could not be loaded
```
Pa11y testing results
```
> pa11y-ci:sitemap
> pa11y-ci --sitemap http://localhost:4000/semester-biology/sitemap.xml --sitemap-exclude "/*.pdf|Knitr-BBS-R.html|paper.*"
The sitemap "http://localhost:4000/semester-biology/sitemap.xml" could not be loaded
```
Pa11y testing results
```
> pa11y-ci:sitemap
> pa11y-ci --sitemap http://localhost:4000/semester-biology/sitemap.xml --sitemap-exclude "/*.pdf|Knitr-BBS-R.html|paper.*"
The sitemap "http://localhost:4000/semester-biology/sitemap.xml" could not be loaded
```
This question asked for nulls to be filtered, but didn't have any null values
Closes #948