Add source_url support in preface.

If a <link/> is present in a preface/coverPage, assume that it is a source URL and store it accordingly. Do not use this in the test for an existing section, as it is quite possible it will change over time.

Based on @jpmckinney's https://github.com/jpmckinney/sayit/commit/33d1bb67dc4e3248e6bc1c77aaec15f7a5a394ae and comments in #354, which this fixes.