chore: adding .editorconfig file

Hi my text  editor is doing stranges things when I'm editing this repo code.
What about fixing some basic rules the `jspm-cli` one (https://github.com/jspm/jspm-cli/blob/ff46dbcada4236603649344913aa543011dcfbc1/.editorconfig) ?

Author: douglasduteil

.editorconfig

@@ -0,0 +1,12 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.md]
+trim_trailing_whitespace = false