If stdout is a pipe, output to the pipe

If you run micro as `micro | cat` for example, micro will disallow
you from saving the file, and when you quit the buffer, the contents
will be sent to the pipe. This allows one to use micro as part of
an interactive unix pipeline.

Closes #1524

internal/util/util.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"os"
@@ -34,6 +35,9 @@ var (
 	// FakeCursor is used to disable the terminal cursor and have micro
 	// draw its own (enabled for windows consoles where the cursor is slow)
 	FakeCursor = false
+
+	// Stdout is a buffer that is written to stdout when micro closes
+	Stdout *bytes.Buffer
 )
 
 func init() {
@@ -46,6 +50,7 @@ func init() {
 	if runtime.GOOS == "windows" {
 		FakeCursor = true
 	}
+	Stdout = new(bytes.Buffer)
 }
 
 // SliceEnd returns a byte slice where the index is a rune index