Bugfix single line program

[ascholerChemeketa]

I just discovered the recent program update doesn't handle the case where the contents of <program><code> does not include any newlines. Samples:

<program>print("Hello")</program>
<program><code>print("Hello")</code></program>

substring-before-last is being used to clean up the trailing whitespace, but that template returns '' if there is no newline. So currently, the code in those samples is dropped.

First commit adds a parma to substring-before-last that allows the caller to ask for the $input back if there is no match. That generates clean diffs for sample-book and sample-article in all formats.

Other commits use that to fix pretext-html and adds a test case to the sample article.

Sorry about this one, I missed that this test case was no where in sample-book or article and thus not getting checked.

[ascholerChemeketa]

Force pushed with new template approach instead of a flag on existing template