Add dump command

[yonaskolb]

Added a new dump command

xcodegen dump

This can be used to output the spec in the following formats with --type:

• yaml
• json
• parsed-yaml (parsed into swift and then back to yaml)
• parsed-json (parsed into swift and then back to json)
• summary (the old output of generate, which has been removed)
• swift-dump (parsed into swift and then a dump() on that

This is useful for:

• migrating to different formats
• integrating with other tooling
• checking the way include and variable expansion is evaluated
• combining specs from multiple include
• testing 2 way parsing

There are opportunities to output graphs of dependencies as well but that will go into a seperate xcodegen graph command