Add :attach command for compose

Allow users to add attachments to emails in the Compose view. Syntax is
:attach <path>, where path is a valid file. Attachments will show up in
the pre-send review screen.

commands/compose/attach.go

@@ -0,0 +1,56 @@
+package compose
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"git.sr.ht/~sircmpwn/aerc/widgets"
+	"github.com/gdamore/tcell"
+	"github.com/mitchellh/go-homedir"
+)
+
+type Attach struct{}
+
+func init() {
+	register(Attach{})
+}
+
+func (_ Attach) Aliases() []string {
+	return []string{"attach"}
+}
+
+func (_ Attach) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (_ Attach) Execute(aerc *widgets.Aerc, args []string) error {
+	if len(args) != 2 {
+		return fmt.Errorf("Usage: :attach <path>")
+	}
+
+	path := args[1]
+
+	path, err := homedir.Expand(path)
+	if err != nil {
+		aerc.PushError(" " + err.Error())
+		return err
+	}
+
+	pathinfo, err := os.Stat(path)
+	if err != nil {
+		aerc.PushError(" " + err.Error())
+		return err
+	} else if pathinfo.IsDir() {
+		aerc.PushError("Attachment must be a file, not a directory")
+		return nil
+	}
+
+	composer, _ := aerc.SelectedTab().(*widgets.Composer)
+	composer.AddAttachment(path)
+
+	aerc.PushStatus(fmt.Sprintf("Attached %s", pathinfo.Name()), 10*time.Second).
+		Color(tcell.ColorDefault, tcell.ColorGreen)
+
+	return nil
+}