support mirror syntax

```perl
# cpanfile
mirror 'http://www.cpan.org';
mirror 'http://backpan.cpan.org';
mirror 'http://example.com/my/darkpan';

...

# usage
use Test::More;
use Module::CPANfile;

my $cpanfile = Module::CPANfile->load;
my $mirrors = $cpanfile->mirrors;

is_deeply $mirrors, [
    'http://www.cpan.org',
    'http://darkpan.cpan.org',
    'http://example.com/my/darkpan',
];

done_testing;
```