Add ability to slice a string given only a start index.

This mimics some languages (for instance C#), where you can ask for a
slice or substring given only a start index to slice from. This permits
easily trimming N many characters from the beginning of a string without
having to know or specify the length in order to make sure the remainder
of the string is returned.

For instance, I noticed in Dave's `earmark` project that he was doing `String.slice(str, ignore, 9999999)` to effectively trim off the number of characters specified by `ignore` and return the remainder of the string. This PR aims to eliminate the necessity to write code like that.