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
It more so that there are a couple open issues that address ascetics.

https://github.com/sass/libsass/issues/452
https://github.com/sass/libsass/issues/593

That are similar and nature whose ascetics (extra whitespaces etc..) will need to be handled at the parser level. Seem logical to keep as much of this logic in one place