Add .editorconfig

Add a proposal for a .editorconfig file (see https://editorconfig.org/).
This is natively supported by a lot of editors, others are supported via
plugins.

This will close #19.

.editorconfig

@@ -0,0 +1,22 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 4
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+max_line_length = 90
+tab_width = 4
+
+[{*.cfg, *.ini, *.html, *.yaml, *.yml}]
+indent_size = 2
+
+[*.json]
+indent_size = 2
+insert_final_newline = false
+
+# trailing spaces in markdown indicate word wrap
+[*.md]
+trim_trailing_whitespace = false