Accept config file

```
$ ukigumo-agent --config=config.pl
```

```perl
# config.pl
{
    work_dir   => File::Spec->tmpdir,
    server_url => 'http://localhost:2828/',
    timeout    => 10,

    ignore_github_tags => 0,
    max_children       => 10,
}
```

ちょっとリファクタもしちゃいましたけど。