Allow custom sequence start

[parzhitsky]

When generating numeric sequences, allow starting them with more than one number:

xrange([1, 1], () => true, ([ last, prelast ]) => last + prelast);
// 1, 1, 2, 3, 5, 8, 13, 21, 34, …

xrange([0, 1, 2, 3, 4], (next) => next < 3, ([ last ]) => last + 1);
// 0, 1, 2
// custom sequence start goes through ("obeys") the predicate

xrange([], (next) => next < 100, ([ last ]) => last + 1);
// (no iterations)
// since sequence start is not defined, the first sequence member is `NaN`, and `NaN < 100` is `false`

[parzhitsky]

Moved to the 3.0 release, since this feature would require a change in error codes, and they will be getting rid of in #71, which is scheduled to 3.0

[parzhitsky]

Blocked by #71