Optionally squash HTML content on to a single line before validation

HTML content usually spans multiple lines, with linebreaks and whitespaces
being insignificant. However, `text?`, `regex?`, `some-text?`, `some-regex?`
apply their validations rigidly. As such, these checks are fragile -- for
instance, if someone changes the template file of the underlying content, the
tests may fail. With this changeset, the user can provide an option
`:unwrap-lines` to make such checks less rigid, and thus, making the tests more
robust.