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/buffer/buffer.go

@@ -5,6 +5,7 @@ import (
 	"bytes"
 	"crypto/md5"
 	"errors"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
@@ -60,6 +61,9 @@ var (
 	BTRaw = BufType{4, false, true, false}
 	// BTInfo is a buffer for inputting information
 	BTInfo = BufType{5, false, true, false}
+	// BTStdout is a buffer that only writes to stdout
+	// when closed
+	BTStdout = BufType{6, false, true, true}
 
 	// ErrFileTooLarge is returned when the file is too large to hash
 	// (fastdirty is automatically enabled)
@@ -82,6 +86,8 @@ type SharedBuffer struct {
 	// Name of the buffer on the status line
 	name string
 
+	toStdout bool
+
 	// Settings customized by the user
 	Settings map[string]interface{}
 
@@ -355,6 +361,10 @@ func (b *Buffer) Fini() {
 		b.Serialize()
 	}
 	b.RemoveBackup()
+
+	if b.Type == BTStdout {
+		fmt.Fprint(util.Stdout, string(b.Bytes()))
+	}
 }
 
 // GetName returns the name that should be displayed in the statusline