Another customizable spacer

Simple customizable spacer.

You can change:

* Outer radius
* Inner radius
* Height
* fn (number of fragments)

The inner rasius is calculated to the inner boundary while the outer radius is the outer one (since meshes are not round ;) )

I used it as a spacer for my kitchen door (printed with PLA).

## UPDATE:

### 23.10.2018

As it turns out i mislabeled the variables - fixed it now. Thank you @Romkslrqusz

Remember:

* A radius is radius is a radius.
* A diameter is a