Silence warning output

## Description
It would be great if you could silence the warning output at CLI, maybe with a flag like

```
catch-exe --no-warn
```

### Steps to reproduce
If you ever do something like
``` C++
WARN("Not a serious problem, but maybe important...")
```
you will always get that output at the console. It would be nice to be able to squelch warning output so you can focus on the actual failures. Hopefully your code only has a handful of errors and only a few warnings, but if it doesn't, this starts to be important.

In my case, I have 100 failures, and 1000 lines of warnings printing to screen, so I lose the forest for the trees.