Allow custom extensions when editing (for easier syntax highlighting) (#1139)

* Create tests and steps for temporary filename suffix
* Have temporary filename suffix be -{template_filename} or .jrnl
* Finalize extension_editor_file
* Make suffix local variable in get_text_from_editor

features/data/configs/editor_markdown_extension.yaml

@@ -0,0 +1,18 @@
+default_hour: 9
+default_minute: 0
+editor: ""
+encrypt: false
+highlight: true
+editor: "vim"
+journals:
+  default: features/journals/editor_markdown_extension.journal
+linewrap: 80
+tagsymbols: "@"
+template: features/templates/extension.md
+timeformat: "%Y-%m-%d %H:%M"
+indent_character: "|"
+colors:
+  date: none
+  title: none
+  body: none
+  tags: none