Add an append_file_content function

Add content to a file resource.

This allows to generate concatenated files on the master side, without managing multiple file resources.

The first argument is the file path. You are responsible for creating the file resource yourself prior to calling the function. This way, you can finely control the file's parameters.

The second argument is the content to be added to the file.

The third (optional) argument is the order, specifying the position in which the content should be added. The default order is '10'. If you specified a content in the original file resource, this content is kept with order '00'

Examples:


    file { '/tmp/target':
      content => "Orig content\n",
    }

    append_file_content('/tmp/target', "hello, ")
    append_file_content('/tmp/target', "world\n")
    append_file_content('/tmp/target', "This is a multi
    line statement
    ", '01')
    append_file_content('/tmp/target', template('foobar/example.erb'), '05')

- *Type* : statement

Doesn't the default order need to be based on the current length of the chunks? Otherwise you can end up with fragments out of order because the later comparison puts them in order based on their contents rather than their insertion order.

```
1.9.3-p484 :002 > [['10', 'z'], ['10', 'a']].sort
 => [["10", "a"], ["10", "z"]]
```

Another solution is to require the order flag, check that it is unique, and not order based on the contents at all.