That particular lens does have an ok macro feature, but there are better lenses for shooting macro. I would suggest maybe a Sigma 90mm f1.8, or if you want to spend the extra money (arguably not better glass) you could get a Nikkor 100mm f1.8. The biggest advantage to a macro vs a telephoto is the focus range. I'm not 100% sure, but I would guess the 18-300 has a focus length of around 4 meters. That's not really close enough for true macro. The other issue you may face with the 18-300 is the inability to stop down small enough to get the look you may desire (Shallow Depth of Field). You may also find that you have to push the ISO a little bit in order to get good exposures leading to grain in your photos.
Here is a little food for thought. The D7100 is a crop body (without going into too much detail that means you can shoot smaller glass with the same range that a full frame sensor produces) so getting a 90mm dedicated macro lens is essentially shooting the same as a 110mm on a full frame body. Now I know the gear heads and Nikkor Nazi's will shoot me for saying this, but you don't need to spend the extra money. It's obviously your call, but IMO I think you will be much happier with a dedicated macro.
