Support "and" in ArithmeticTimePlugin to behave like "+" sign

[ramiabughazaleh]

Thank you for NaturalTimeParser 0.2.0.0.

I would like to be able to use "and" instead of the "+" plus sign when describing a specific date/time.

For example, "1 year ago and 5 days ago" should be parsed the same as "1 year ago + 5 days ago".

Thank you!