MINOR: Add .editorconfig

This will help ensure that tabs/spaces/trailing-space/newlines are consistent.

This file is based on the one from silverstripe-framework

.editorconfig

@@ -0,0 +1,24 @@
+# For more information about the properties used in
+# this file, please see the EditorConfig documentation:
+# http://editorconfig.org/
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 4
+indent_style = tab
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.md]
+trim_trailing_whitespace = false
+
+[*.yml]
+indent_size = 2
+indent_style = space
+
+# Don't perform any clean-up on thirdparty files
+
+[thirdparty/**]
+trim_trailing_whitespace = false
+insert_final_newline = false