Add .editorconfig file

EditorConfig (http://editorconfig.org/) has become the
de facto standard for defining the use of white space in
project files. Code editors then choose the right
settings automatically when a file is edited.

https://bugzilla.gnome.org/show_bug.cgi?id=786620

.editorconfig

@@ -0,0 +1,16 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+
+[*.{vala,vapi,gs,metadata}]
+indent_style = tab
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[Makefile.am]
+indent_style = tab
+
+[meson.build]
+trim_trailing_whitespace = true