support for cmd option

[brewster1134]

since coffeelint has an executable binary, it would be great to be able to customize how it is called (similar to guard-rspec). i want to have guard-coffeelint listed in my package.json as a dependency, however that doesn't mean it will be installed globally, and therefore not accessible in my $PATH. so to be able to run the binary included in my local project node_modules, and be able to run that, would be very helpful. something like...

guard :coffeelint, cmd: 'npm bin coffeelint' do
end