.osx: Use `sudo` for `systemsetup`

Closes #415.
.osx: Disable the all-too-sensitive backswipe on Chrome

Closes #416.
Update URLs
.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.
.functions: Add SNI support in `getcertnames`
Update URLs
.functions: Improve `getcertnames`
.gitconfig: Add alias to merge GitHub pull request

Ref. https://help.github.com/articles/checking-out-pull-requests-locally

Closes #422.
Merge branch 'master' of https://github.com/mathiasbynens/dotfiles

* 'master' of https://github.com/mathiasbynens/dotfiles:
  .gitconfig: Add alias to merge GitHub pull request
  .functions: Improve `getcertnames`
  Update URLs
  .functions: Add SNI support in `getcertnames`
  .vimrc: Treat `.md` files as Markdown
  Update URLs
  .osx: Disable the all-too-sensitive backswipe on Chrome
  .osx: Use `sudo` for `systemsetup`