.osx: Use `sudo` for `systemsetup`

Closes #415.

Conflicts:
	.osx
.osx: Disable the all-too-sensitive backswipe on Chrome

Closes #416.
.vimrc: Treat `.md` files as Markdown

`filetype` is set implicitly, without using setfiletype, because `*.md` is already associated with Modula-2 files and `setfiletype` doesn’t allow `filetype` to be set twice.

`setlocal` is used to apply a new `filetype` value for each Markdown file as it is being added to the buffer.

Fixes #418. Closes #419.