Add STDNIN placeholder

In order to forward the original hook stdIn to other scripts the
Cap'n now provides a STDIN placeholder.
For example forwarding the stdIn to existing git-lfs hooks would
look like this

  {"action": "echo {$STDIN} | hooks/git-lfs/pre-push"}

hooks/placeholder/placeholder.go

@@ -28,6 +28,9 @@ var (
 			files, _ := aContext.Repository().StagedFiles()
 			return &FileList{name: "STAGED_FILES", context: aContext, files: files}
 		},
+		"STDIN": func(aContext *app.Context) Replacer {
+			return &StdIn{context: aContext}
+		},
 	}
 )
 

hooks/placeholder/stdin.go

@@ -0,0 +1,18 @@
+package placeholder
+
+import (
+	"github.com/captainhook-go/captainhook/hooks/app"
+	"github.com/captainhook-go/captainhook/io"
+)
+
+type StdIn struct {
+	context *app.Context
+}
+
+func (r *StdIn) Replacement(options map[string]string) string {
+	esc := "'"
+	if !io.AnswerToBool(io.MappedStringOrDefault(options, "escaped", "true")) {
+		esc = ""
+	}
+	return esc + r.context.IO().Option("input", "") + esc
+}

hooks/placeholder/stdin_test.go

@@ -0,0 +1,49 @@
+package placeholder
+
+import (
+	"github.com/captainhook-go/captainhook/configuration"
+	"github.com/captainhook-go/captainhook/git"
+	"github.com/captainhook-go/captainhook/hooks/app"
+	"github.com/captainhook-go/captainhook/io"
+	"testing"
+)
+
+func TestStdin(t *testing.T) {
+	argMap := map[string]string{}
+	optMap := map[string]string{"input": "foo bar"}
+	expected := "'foo bar'"
+
+	config := configuration.NewConfiguration("foo", false)
+	repo, _ := git.NewRepository(".git")
+	ctx := app.NewContext(
+		io.NewDefaultIO(io.NORMAL, optMap, argMap),
+		config,
+		repo,
+	)
+	opts := map[string]string{}
+	placeholder := &StdIn{context: ctx}
+	result := placeholder.Replacement(opts)
+	if result != expected {
+		t.Errorf("Replacement didn't work, got: %s, want: %s.", result, expected)
+	}
+}
+
+func TestStdinUnescaped(t *testing.T) {
+	argMap := map[string]string{}
+	optMap := map[string]string{"input": "foo bar"}
+	expected := "foo bar"
+
+	config := configuration.NewConfiguration("foo", false)
+	repo, _ := git.NewRepository(".git")
+	ctx := app.NewContext(
+		io.NewDefaultIO(io.NORMAL, optMap, argMap),
+		config,
+		repo,
+	)
+	opts := map[string]string{"escaped": "false"}
+	placeholder := &StdIn{context: ctx}
+	result := placeholder.Replacement(opts)
+	if result != expected {
+		t.Errorf("Replacement didn't work, got: %s, want: %s.", result, expected)
+	}
+}