I didn't want to install a package just for that purpose so I ended up using this in my init.coffee:
spawn = require('child_process').spawn
atom.commands.add 'atom-text-editor', 'open-terminal', ->
file = atom.workspace.getActiveTextEditor().getPath()
dir = atom.project.getDirectoryForProjectPath(file).path
spawn 'mate-terminal', ["--working-directory=#{dir}"], {
detached: true
}
With that, I could map ctrl-shift-t to the open-terminal command and it opens a mate-terminal.