Added support for specifying between/notBetween formula in an array.

Modified conditional_formatting_rule.rb to accept an array or string for formula. This is converted to consecutive <formula>val</formula> tags if present. Added test. All tests pass.

I thought of minimum/maximum, or formula1 or formula2, but it seemed overbearing. 

Tried validation that restricted arrays to :cellIs and :between/:notBetween rules. But, based on my tests with Excel and LibreOffice, second <formula> entries are ignored, so the only thing that matters is that the array, if passed, has strings in it. 

Added an example also.