Alter the invert function to accept numbers, for native CSS invert

This fixes a minor bug, where if you try and use "-webkit-filter: invert(100%);", the invert function will fail since it is expecting a colour. (See bug on Sassmeister here: https://gist.github.com/iamcarrico/8f11aacc54391db152a6)This is already changed in [Ruby Sass](https://github.com/sass/sass/blob/stable/lib/sass/script/functions.rb#L1311), and I did some alterations to allow invert to have numbers or colours. If a number is passed on, then it will return as a string with the number inside. 

Thoughts? My C++ is ... rusty. So there might be a better way.