.editorconfig: Set charset for all files to UTF-8

Closes GH-620.

.editorconfig

@@ -1,15 +1,14 @@
 root = true
 
 [*]
+charset = utf-8
 insert_final_newline = true
 trim_trailing_whitespace = true
 
 [*.yaml]
-charset = utf-8
 indent_size = 2
 indent_style = space
 
 [Makefile]
-charset = utf-8
 indent_size = 4
 indent_style = tab