Correcting link boundary checking

This addresses #434 where it was noted that links flowing across multiple lines weren't checking boundaries right.

I believe this properly works for links at my position in a given `TextBlock`. It deals with links on single and multiple lines, and doesn't make any assumptions about the presence of other links within the text.

I've run it against `samples/class-book-adjusted.rb` (where I'd noted some issues on flowing links before), `samples/simple-manual.rb` which has a lot of single-line links, and a sample based on #434.

Link on!