Shorthand only flags

I wanted flags that can only be specified as `-f` and not `--f`. Mainly because it allows for the help to look better.
I think this type of flag is mostly useful for bool flags, but I added convenience methods/functions for all flag types by adding `S` to the name: `BoolS`.
For `BoolS("f", false, "blah")` the help is: `      -f=false: blah`.