Add implicit dependencies information

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Check this tweet for context: https://twitter.com/NeoNacho/status/846346465911951360

At the moment the graph only has information from the "Link Binary With Libraries" build phase. It'll be a good idea to add dependencies that come from xcconfig files and project Build Settings, e.g.: -framework, -l.

Notes:

• This settings depend on the target configuration, so we'll have to decide if we either pass the configuration as a parameter or we do something else, like flatten the results.
• Check if Xcodeproj::Project.build_settings returns resolved build settings (if there's an xcconfig file attached to the project), otherwise we would have to parse the xcconfig file attached to the target's build configuration.
• Xcodeproj::Config provides support for parsing xcconfig files.

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Let's wait for xcodeproj 1.5 to make a release including #10