Support arithmetic expressions

[doits]

Thanks for this great library!

I just noticed the following:

JMESPath.search('`2` * `2`', '')
# => unexpected token star (JMESPath::Errors::SyntaxError)

I guess arithmetic expressions are not supported by this gem?

If so, maybe they could be added?

[doits]

Ah, sorry, now I see this is a "community extension". I guess this comes down the to support the "JMESPath Community specifications".