Rework config/flags (fixes #13)

usage.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"io"
+	"text/tabwriter"
+)
+
+func optionTable(w io.Writer, rows [][]string) {
+	tw := tabwriter.NewWriter(w, 2, 4, 2, ' ', 0)
+	for _, row := range rows {
+		for i, cell := range row {
+			if i > 0 {
+				tw.Write([]byte("\t"))
+			}
+			tw.Write([]byte(cell))
+		}
+		tw.Write([]byte("\n"))
+	}
+	tw.Flush()
+}
+
+func usageFor(fs *flag.FlagSet, usage string) func() {
+	return func() {
+		var b bytes.Buffer
+		b.WriteString("Usage:\n  " + usage + "\n")
+
+		var options [][]string
+		fs.VisitAll(func(f *flag.Flag) {
+			var dash = "-"
+			if len(f.Name) > 1 {
+				dash = "--"
+			}
+			var opt = "  " + dash + f.Name
+
+			if f.DefValue != "false" {
+				opt += "=" + f.DefValue
+			}
+
+			options = append(options, []string{opt, f.Usage})
+		})
+
+		if len(options) > 0 {
+			b.WriteString("\nOptions:\n")
+			optionTable(&b, options)
+		}
+
+		fmt.Println(b.String())
+	}
+}