Output leading zeros on numbers as seen in input (Fixes #550)

This fixes #550 and some other related issues!

To do this I had to add a status bool to `Number` to preserve the state that we have parsed!
Turned out to be trickier than anticipated, but it should now be in line with ruby sass!

I initially implemented that status flag on all number representations down to the C interface, but decided to drop it for now as it would mean a small change in the API and I [added this a local branch] [1] for now.

[1]: https://github.com/mgreter/libsass/tree/issue/550-c-interface
Hmm, It would need more changes as I would to add the `zero` flag to the Textual type too.
IMO it's ok that way as other code in eval also parses the lexed input for more information!
Anything speaking strictly against doing it this way?