add is_enum to insure that bit shifts are not mistaken for io stream operators

[lintondf]

The following sketch fails with the baseline PrintEx and compiles with this pull request

#include <PrintEx.h>

enum pin_t {
D0 = 0x61,
};

void setup() {
// libraries\PrintEx\src/lib/TypeTraits.h:239:22: error: base type 'pin_t' fails to be a struct or class type
int reg = D0 >> 4;
}

void loop() {
// put your main code here, to run repeatedly:

}