feat(main): implement []string flag valuer

Implement flag.Value to automnatically split strings into an array at
their spaces.

Change-Id: Ia9139e23d74a30acfc74cb65935bb7fc2b322aec

internal/flag/flag.go

@@ -0,0 +1,34 @@
+package flag
+
+import (
+	"fmt"
+	"strings"
+)
+
+// A StringsValue is a command-line flag that interprets its argument
+// as a space-separated list of strings.
+type StringsValue []string
+
+// Set implements the flag.Value interface by spliting the provided string at
+// spaces.
+func (v *StringsValue) Set(s string) error {
+	if s == "" {
+		*v = []string{}
+		return nil
+	}
+
+	*v = strings.Fields(s)
+
+	return nil
+}
+
+// Get implements the flag.Getter interface by returning the contents of this
+// value.
+func (v *StringsValue) Get() interface{} {
+	return []string(*v)
+}
+
+// String returns a string
+func (v *StringsValue) String() string {
+	return fmt.Sprintf("%s", *v)
+}

internal/flag/flag_test.go

@@ -0,0 +1,29 @@
+package flag_test
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/terinjokes/bakelite/internal/flag"
+)
+
+func TestStringsValue(t *testing.T) {
+	tests := []struct {
+		v    string
+		want []string
+	}{
+		{"", []string{}},
+		{"a b c", []string{"a", "b", "c"}},
+		{"foo bar baz", []string{"foo", "bar", "baz"}},
+		{"foo  bar  baz", []string{"foo", "bar", "baz"}},
+	}
+
+	for i, tt := range tests {
+		got := flag.StringsValue([]string{})
+		got.Set(tt.v)
+
+		if diff := cmp.Diff(tt.want, got.Get()); diff != "" {
+			t.Errorf("#%d: manifest differs. (-got +want):\n%s", i, diff)
+		}
+	}
+}