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.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.
Brewfile: Remove installs

Remove `ack` in favor of using `ag`. Remove JavaScript virtual machines except for Node.js since I will probably never use them.
Brewfile: Comment out installs

Comment out installs that aren't immediately useful to me but seem interesting enough to keep in mind in the future.
Brewfile: Group programming language installs
Brewfile: Install `ag`
Brewfile: Install Go and Python
Brewfile: Install Laravel dependencies
Brewfile: Install `hub`
Brewfile: Install `heroku-toolbelt`
Brewfile: Install databases
Caskfile: Remove installs

Remove apps I doubt I'm ever going to use. Remove The Unarchiver install in favor of installing it from the App Store.
Caskfile: Comment out installs

Comment out installs that aren't immediately useful to me but seem interesting enough to keep in mind in the future.
Caskfile: Install Quick Look plugins
Caskfile: Install work utility apps
Caskfile: Install entertainment apps
Caskfile: Install LastPass
Caskfile: Install paperless utility apps
Caskfile: Install AppCleaner
Caskfile: Install Copy