Expand isnumeric() function

Currently, the isnumeric() functions returns true only if all characters in the string are decimal digits (i.e. 0 - 9). It would be helpful to expand out the function to take in an argument that tells it to allow other type of numeric digits like hexadecimal. 

Ex. 
```c++
String num("0x3E8"); // Valid hexadecimal for 1000

REQUIRE(num.isnumeric(bool hex=true));
```