allow path repository failing

[shyim]

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 :)

[JoshuaBehrens]

Such a good thing for project templates. Always annoying to not have them at the start but end up setting these up anyways.

[Seldaek]

Yeah that sounds reasonable to me if you like to send a PR.