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.

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#### Question for pull request reviewers 

I personally think `:unwrap-lines` is a better default, while providing `:keep-as-is` as an option. However, for the sake of current users of the library, I kept the default behavior as is. Let me know what you think.