EPS files not created

When I add EPS file creation to the example usage program on the README.md page, I get errors like this:
```
ERROR: could not create EPS file for: usage_example, switch | exit status 1
```
I added this line to the example usage program before the `cad.Draw()` line:
```
cad.Result.Formats = append(cad.Result.Formats, []string{"eps"}...)
```