add file type

This is my first commit to configspec. Could you review this?

I added `file` type to enable

```ruby
describe file('filename') do
  it { should contain 'contents' }
end
```

which appends `contents` to `filename` file. 

I very wondered the specification of configspec, but tried to match with serverspec http://serverspec.org/resource_types.html#file. What do you think of this?

This pull request requires https://github.com/serverspec/specinfra/pull/32, which adds `append` command. Without a release of specinfra, the test of configspec will fail.