Allow value(json_pointer) to work with arrays as documented

[falbrechtskirchinger]

As discovered here, value(json_pointer) throws an exception, when accessing an array. This deviates from the documented behavior:

try {
   return at(ptr);
} catch(out_of_range) {
   return default_value;
}

Allow value(json_pointer) to work with arrays, update documentation, and update unit tests.

[nlohmann]

Would it make sense to also have an overload for integral types just like at or operator[]?

[falbrechtskirchinger]

Right, I was going to look at that as well.

Also, if needed, that is probably better suited for the planned "parity PR". I've identified a few functions with "missing" overloads. Adding it to the list.

[coveralls]

Coverage remained the same at 100.0% when pulling 6724d9450fa8e6d8301d426111799c4084281911 on falbrechtskirchinger:value-json_ptr into f4658de2701f2290baf076bbbd8540260015518c on nlohmann:develop.