Expose collection attribute from Document role

[jmmills]

Make Meerkat::Role::Document::_collection attribute non-private so that objects can be "clone aware".
For example:

package My::Template;
use Moose;
with 'Meerkat::Role::Document;

has latest => (...boolean..);
has version => (...number..);
has content => (...string...);

sub as_hash { ... }

sub update_content {
    my ($self, $content) = @_;
    my $clone = $self->as_hash;

    $self->update_set( content => $content );
    $self->update_inc( version => 1 );
    $self->sync;

    $clone->{latest} => 0;
    $self->collection->create( %{ $clone } ); # as $self->_collection

    return $self;
} 

package main;
say $meerkat->collection('Template')
            ->find_one({latest => 1})
            ->update_content('I like fluffy bunnies')
            ->version;