Support short/long flags

[ogerardin]

Most command-line tools support a short and a long form for flags, e.g. "-p" or "--port".
Currently confita only supports a single flag per member.

Suggestion: add a "short" sub-tag and handle it as a synonym of the default name. Also update help usage generation to reflect this.

type Configuration struct {
	Port          int    `config:"port,short=p"`
}

[steamrolla]

This is ALMOST done... in flags_test.go we are missing a couple types from the struct. Should have this done today, and, this should complete the enhancement.

[steamrolla]

should be good.