Handle whitespace correctly

The Emacs Editorconfig [suddenly added support for
`insert_final_newline =
false`](editorconfig/editorconfig-emacs@c707d8d),
making me realize I didn't actually want that enabled. I never noticed
this behavior because VS Code's Editorconfig extension [apparently
doesn't support this
setting](editorconfig/editorconfig-vscode#349).

`trim_trailing_whitespace = true` is a nice convenience
feature (except in Markdown, where I kept it disabled). Not sure why I
had it disabled.

.editorconfig

@@ -5,9 +5,12 @@ indent_style = tab
 indent_size = 2
 end_of_line = lf
 charset = utf-8
-trim_trailing_whitespace = false
-insert_final_newline = false
+trim_trailing_whitespace = true
+insert_final_newline = true
 
 [*.yml]
 indent_style = space
-indent_size = 2
+indent_size = 2
+
+[*.md]
+trim_trailing_whitespace = false