Use a constant for finding whitespace

[paazmaya]

While cleaning up my forks, I found this commit https://github.com/paazmaya/sass2scss/commit/56131397ea475db70de0ccb6cab7461c9cd75f00, which might be used to reduce the change for typing error, by using a constant in the places where whitespace is looked for.

The commit also contains using a local variable, instead of calling string position several times. There might be a reason for doing so, but if not, how about using the local variable.

I can make a PR out of that, it useful.

[mgreter]

Sure, if it is based on the latest master I'll merge it in!