"Short" args must be single letter to work, but not checked

[rpavlik]

Discovered "the hard way" in a Catch2-using project that while you are allowed to add flags like ["-abc"] for an Opt, trying to actually specify it will never work - you'll just get:

Error(s) in input:
  Unrecognised token: -a

I imagine it's doing this because it wants to support combining short options. However, in that case, there should probably be a check at parser-creation time for single-letter usage with short option symbol.

[grafikrobot]

I addressed this in the Lyra lib by making ["-abc"] equate to ["-a"]["-b"]["-c"]. As it mirrors the usage and avoid exceptions. Change is available from this version bfgroup/Lyra@d4db7b8