Regularize whitespace; add editorconfig

- Change the few files that use leading tabs into leading spaces.
  This way they match the other files in the repo.
- Add an EditorConfig file so that supported editors will automatically
  use spaces instead of tabs.
- (Extra) Add some common editor backup-file extensions to .gitignore.

.editorconfig

@@ -0,0 +1,7 @@
+# see https://editorconfig.org for the format of this file
+root = true
+
+[*]
+end_of_line = LF
+indent_style = space
+indent_size = 4

.gitignore

@@ -3,3 +3,8 @@ http/images
 http/*.html
 http/favicon.ico
 http/robots.txt
+
+# Editor and other backup files
+*.swp
+*~
+~*