Allow custom arg validations

[kbknapp]

One should be allowed to specify a custom validation function which will be run against an argument value.

From a birds eye view:

let app = App::new("test")
              .arg(Arg::with_name("opt")
                     .long("long")
                     .takes_value(true)
                     .validator(|val| { 
                         if val.len() == 5 { 
                             Ok(val) 
                         } else { 
                             Err("--long must be 5 characters long") 
                         }
                     })
               );  

If possible I may also look at providing some pre-conceived validator functions which would could be used like so:

let app = App::new("test")
              .arg(Arg::with_name("opt")
                     .long("long")
                     .takes_value(true)
                     .validator( ArgValidators::U8 )
               );