editorconfig: add HTML entry

While there aren't many HTML files, use consistent formatting
with 2 space indentation & 99 columns.

.editorconfig

@@ -59,4 +59,13 @@ trim_trailing_whitespace = true
 insert_final_newline = true
 indent_style = tab
 indent_size = 4
-max_line_length = 120
+max_line_length = 120
+
+# HTML
+[*.html]
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+indent_style = space
+indent_size = 2
+max_line_length = 99