Add substring method to string class

[WoodchuckXL]

Adds the String substring(int begin, int end) method to the string class.
Takes two integer arguments for the begin (inclusive) and end (exclusive) indices of the substring and returns a new substring of the original string between those indices. If the begin or end indices are outside of the bounds of the string, they will be clamped to beginning or end respectively. If the indices don't form a valid substring, substring returns an empty string.

[softmattertheory]

This is a really useful contribution; thanks for including the test as well!