add editorconfig file to give hints to editors

We use an unusual indent of three spaces. Let's give editors a hint
about that.

.editorconfig

+# EditorConfig configuration for htop
+# http://EditorConfig.org
+
+# top-most EditorConfig file
+root = true
+
+# Unix-style newlines with a newline ending every file, utf-8 charset
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+
+# match C source and header files, set indent to three spaces
+[*.{c,h}]
+indent_style = space
+indent_size = 3