Add optional strict parameter to isPrime typing.

I was checking out the source to see the difference between isPrime and isProbablePrime when I stumbled upon this undocumented 'strict' parameter.

Author: silentsilas

BigInteger.d.ts

@@ -200,7 +200,7 @@ declare namespace bigInt {
         /**
          * Returns true if the number is prime, false otherwise.
          */
-        isPrime(): boolean;
+        isPrime(strict?: boolean): boolean;
 
         /**
          * Returns true if the number is very likely to be prime, false otherwise.