Do not show the Markdown Editor for txt files

Fixes #55

lib/screens/note_editor.dart

@@ -111,6 +111,12 @@ class NoteEditorState extends State<NoteEditor> {
           break;
       }
     }
+
+    // Txt files
+    if (note.fileFormat == NoteFileFormat.Txt &&
+        editorType == EditorType.Markdown) {
+      editorType = EditorType.Raw;
+    }
   }
 
   @override
@@ -190,7 +196,7 @@ class NoteEditorState extends State<NoteEditor> {
     var newEditorType = await showDialog<EditorType>(
       context: context,
       builder: (BuildContext context) {
-        return NoteEditorSelector(editorType);
+        return NoteEditorSelector(editorType, _note.fileFormat);
       },
     );
 

lib/widgets/note_editor_selector.dart

@@ -1,17 +1,20 @@
 import 'package:flutter/material.dart';
 import 'package:font_awesome_flutter/fa_icon.dart';
 import 'package:font_awesome_flutter/font_awesome_flutter.dart';
+import 'package:gitjournal/core/note.dart';
 import 'package:gitjournal/screens/note_editor.dart';
 
 class NoteEditorSelector extends StatelessWidget {
   final EditorType currentEditor;
+  final NoteFileFormat fileFormat;
 
-  NoteEditorSelector(this.currentEditor);
+  NoteEditorSelector(this.currentEditor, this.fileFormat);
 
   @override
   Widget build(BuildContext context) {
     var list = Column(
       children: <Widget>[
+        if (fileFormat != NoteFileFormat.Txt)
           _buildTile(
             context,
             EditorType.Markdown,