Add implicit helpers for numeric arguments

Similar to RangedArgs this captures logic for handling numeric arguments.  I could also imagine this logic living on Args itself.  
1. I'm not sure you are getting any mileage out of this. Couldn't you just inline the parsing in the methods below? You don't expose a generic way to use this.

2. NumberFormatException is too specific here. I would have gone with:
```scala
import scala.util.NonFatal
try f(key, value) catch {
  case NonFatal(_) => throw ArgsException(s"Invalid value ${value} for -- ${key}")
}
```