Allow editing multiline prompts with external text editor (#429)

- Adds a new `Preferences` struct to the config, initially only containing `Editor: bool (default false)`.
  This struct will be serialized to configs once there is a first tea induced change to the config (eg `tea login default <name>` or `tea login add`).
- Use external editor for all multiline prompts if preferred.

We already had a function for starting a texteditor for diff reviews; it does not really make sense to replace it with `survey.Editor`, as there is a big interface mismatch: survey expects strings as inputs, while our diff functions operate on files,

fixes #424

Co-authored-by: Norwin <git@nroo.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/429
Reviewed-by: Andrew Thornton <art27@cantab.net>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

cmd/comment.go

@@ -9,13 +9,15 @@ import (
 	"io/ioutil"
 	"strings"
 
-	"code.gitea.io/tea/modules/interact"
-
-	"code.gitea.io/sdk/gitea"
 	"code.gitea.io/tea/cmd/flags"
+	"code.gitea.io/tea/modules/config"
 	"code.gitea.io/tea/modules/context"
+	"code.gitea.io/tea/modules/interact"
 	"code.gitea.io/tea/modules/print"
 	"code.gitea.io/tea/modules/utils"
+
+	"code.gitea.io/sdk/gitea"
+	"github.com/AlecAivazis/survey/v2"
 	"github.com/urfave/cli/v2"
 )
 
@@ -54,7 +56,11 @@ func runAddComment(cmd *cli.Context) error {
 			body = strings.Join([]string{body, string(bodyStdin)}, "\n\n")
 		}
 	} else if len(body) == 0 {
-		if body, err = interact.PromptMultiline("Content"); err != nil {
+		if err = survey.AskOne(interact.NewMultiline(interact.Multiline{
+			Message:   "Comment:",
+			Syntax:    "md",
+			UseEditor: config.GetPreferences().Editor,
+		}), &body); err != nil {
 			return err
 		}
 	}