nitpick: missing input validation for `margin`

[sanga]

I fat-fingered the value for margin and got the following.

wmfocus ---halign=center --valign=center --font=Hack:140 --margin=5-    2.007s
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { message: "\u{1b}[1;31merror:\u{1b}[0m Invalid value: The argument \'5-\' isn\'t a valid value", kind: ValueValidation, info: None }', src/utils.rs:221:18
stack backtrace:
   0: <unknown>
   1: <unknown>
   2: <unknown>
   3: <unknown>
   4: <unknown>
   5: <unknown>
   6: <unknown>
   7: <unknown>
   8: <unknown>
   9: <unknown>
  10: <unknown>
  11: <unknown>
  12: __libc_start_main
  13: <unknown>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

The stack is basically pointless, sorry, I just installed this from AUR. Presumably the bug is pretty obvious here however (though I haven't actually looked at the code).

[svenstaro]

Good of you to point this out. Reminds me to port this to structopt.

[svenstaro]

Could you check whether this is now adequately fixed for you as of 641b42b?

[sanga]

Works perfectly. Thank you!