Add :save and :pipe commands to viewer

* :save takes a path and saves the current message part to that location
* :pipe is the same as pipe on the account page, but uses the current
  message part rather than the whole email (ie :pipe gzip -d)
* Refactored account:pipe and extracted common pipe code to
  commands.util.QuickTerm
* Added helper command aerc.PushError

commands/msgview/save.go

@@ -0,0 +1,59 @@
+package msgview
+
+import (
+	"encoding/base64"
+	"errors"
+	"io"
+	"mime/quotedprintable"
+	"os"
+	"time"
+
+	"git.sr.ht/~sircmpwn/aerc/widgets"
+	"github.com/mitchellh/go-homedir"
+)
+
+func init() {
+	register("save", Save)
+}
+
+func Save(aerc *widgets.Aerc, args []string) error {
+	if len(args) < 2 {
+		return errors.New("Usage: :save <path>")
+	}
+
+	mv := aerc.SelectedTab().(*widgets.MessageViewer)
+	p := mv.CurrentPart()
+
+	p.Store.FetchBodyPart(p.Msg.Uid, p.Index, func(reader io.Reader) {
+		// email parts are encoded as 7bit (plaintext), quoted-printable, or base64
+		switch p.Part.Encoding {
+		case "base64":
+			reader = base64.NewDecoder(base64.StdEncoding, reader)
+		case "quoted-printable":
+			reader = quotedprintable.NewReader(reader)
+		}
+
+		target, err := homedir.Expand(args[1])
+		if err != nil {
+			aerc.PushError(" " + err.Error())
+			return
+		}
+
+		f, err := os.Create(target)
+		if err != nil {
+			aerc.PushError(" " + err.Error())
+			return
+		}
+		defer f.Close()
+
+		_, err = io.Copy(f, reader)
+		if err != nil {
+			aerc.PushError(" " + err.Error())
+			return
+		}
+
+		aerc.PushStatus("Saved to "+target, 10*time.Second)
+	})
+
+	return nil
+}