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.

scanner.go

@@ -40,6 +40,10 @@ func (t Token) String() string {
 	}
 }
 
+func (t Token) IsEOL() bool {
+	return t.Type == EOLToken
+}
+
 func (t Token) IsAny(types ...TokenType) bool {
 	for _, typ := range types {
 		if t.Type == typ {