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