Tracing parser (#11)

* Add tracing to the parser.

* Synthesize a --help flag.

* Parsing now occurs in multiple phases.

1. Reset target.
2. Parse command-line into a "trace" (no values are written to target).
3. Apply traced, parsed values to the target fields.

This is another step in facilitating context-sensitive help and
completion.

* Detect duplicate flags.

help.go

@@ -9,6 +9,9 @@ const defaultHelp = `{{- with .Application -}}
 usage: {{.Name}}
 
 {{.Help}}
+{{range .Flags}}
+--{{.Name}}
+{{end}}
 
 {{- end -}}
 `