Add editorconfig.

src/.editorconfig

@@ -0,0 +1,19 @@
+
+root = true
+
+# Default to reasonable settings
+[*]
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+# Ignore external libs
+[/external/**]
+insert_final_newline = unset
+trim_trailing_whitespace = unset
+indent_style = unset
+indent_size = unset
+
+# CMake scripts, including those in the top level of external which we wrote
+[CMakeLists.txt,.cmake,/external/*.cmake]
+indent_style = tab
+indent_size = 4