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

# By Jevgeni Zelenkov (1) and Mathias Bynens (1)
# Via Mathias Bynens
* 'master' of https://github.com/mathiasbynens/dotfiles:
  .vimrc: Treat `.md` files as Markdown
  Update URLs