Fix reference-style links in first paragraph

This fixes issue #5.

Instead of finding the first paragraph from the Markdown source, convert it to HTML and then find the first `<p>` tag using cheerio.  Two new tests are also included to ensure reference-style links are converted to anchor tags correctly and to ensure that the excerpt is the first Markdown paragraph, not a code block.

Also, whitespace is now trimmed from excerpts before adding them to a file.