Add support for upcoming source_provider plugin hook

[amorde]

Here's an initial implementation that supports the yet-to-be-completed plugin source_provider hook as described in CocoaPods/CocoaPods#3792

Some tests should probably be made for the new functionality, but for now I have successfully tested it on my machine using my branch of cocoapods that includes the new plugin hook.

Example:

plugin 'cocoapods-repo-svn', {
    :sources => [
        "https://svn.myrepo.com/private-specs"
    ]
}

$ pod install
Updating spec repo `private-specs`
Updating '.':
At revision 12345.
Analyzing dependencies
Downloading dependencies
...

About the SVN executable & backticks.. I kept getting errors such as "Unknown subcommand: 'up --non-interactive --trust-server-cert'" from svn when using the svn!('up --non-interactive --trust-server-cert') syntax.

Let me know what you think!

[dustywusty]

This looks great @amorde!

Waiting on CocoaPods/CocoaPods#3792 to be merged

🍰 🍬 🍰

[amorde]

Awesome!

[eseilnacht]

+1