Let me point at my own excludesfile

I'd like to point to my own excludesfile. This PR relies on the recent hiera work to allow me to override the defaults.

p.s. This is my first contribution to a Boxen module, if I've done something wrong I'd love to know about it so I can do better next time.

p.p.s. I tried to write tests, but my unfamiliarity with testing Puppet modules is making it difficult. Any guidance here gratefully received - here's the test I tried to write in `./spec/classes/git_spec.rb`:

```ruby
context 'when git::global_excludesfile fact is set' do
  let(:facts) do
    default_test_facts.merge({
      :boxen_home    => boxenhome,
      :boxen_repodir => repodir,
      :'git::global_excludesfile' => '/some/other/file',
    })
  end

  it do
    should contain_git__config__global('core.excludesfile').with({
      :value => '/some/other/file'
    })
  end
end
```