Allow using short name only for flags and change '%{long}' in flag translation to '%{name}'

[DannyBen]

Prior to this PR, specifying a short flag only, generated a broken script.

flags:
- short: -1
  help: Same as --tail 1

This PR fixes it.

Note that for those using custom translations, there is one change. Instead of %{long}, use %{name} in the flag_requires_an_argument key:

- flag_requires_an_argument: "%{long} requires an argument: %{usage}"
+ flag_requires_an_argument: "%{name} requires an argument: %{usage}"