Negative price amount

[rennerocha]

This PR fixes #29 . It allows the return of negative amount values when needed.

[Gallaecio]

I like a lot of things about the change, but I don’t think it is a good idea to modify the signature of a public function like extract_price_text like this. It may be better to create a new function based on extract_price_text instead. And maybe make it private to avoid an issue like this in the future.

[rennerocha]

@Gallaecio ,

Besides the fact that extract_price_text seems to be a public function, it is never mentioned in the docs, so only users that read the source code of the library would be using it in their project. In my opinion I may consider it as a private function (just rename it to _extract_price_text would be an acceptable solution).

However I understand your concern about changing something that may be backward incompatible (specially because I changed the return value of the function), so I renamed it to _extract_price_amount and made extract_price_text as before just using this new private function.

Please let me know if it is better now!

Thanks!

[Gallaecio]

So we are determining if the amount is negative or positive before we have extracted the amount. I think this may be a problem, for example once #5 is merged, where - 2 items at 24,00 € could parse the price as -24€ (I did not check, though).

[rennerocha]

You broke my tests! :-(
You are right. This scenario will fail doing that way. However I don't see how to identify it after extracting the amount. I will include it to the tests and try to solve it.