allow path repository failing

I have optional path repositories and when they don't exist I get:

```
0.078 In PathRepository.php line 163:
0.078
0.078 The `url` supplied for the path (custom/plugins/*) repository does not exis
0.078 t
0.078
```

What I would suggest having a `optional` flag, so when there is an error, skip it

```
{
"type": "path",
"url": "custom/plugins/*",
"options": {
"symlink": true,
"optional": true
}
}
```

It's straightforward to integrate, looking for feedback before I make the PR :)