Add .editorconfig

This makes it easier for contributors to follow encoding, line ending, indentation, and other settings.

http://editorconfig.org/

Closes gh-1101

.editorconfig

@@ -0,0 +1,9 @@
+root = true
+
+[*]
+charset = utf-8
+indent_style = space
+indent_size = 4
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true