feature - ability to specify additional source/include folders to add to project

[ofTheo]

This is just the command line PG changes necessary to add other source / includes folders to a project outside of the project directory.

PR adds a --source or -s argument to the command line PG and lets you specify both absolute or relative source or include paths to add to the project. eg:

/projectGenerator -o"../../../../" -s"../../../../SharedCode" -p"osx"  ../../../../apps/myApps/testPG1

This is super helpful for Visual Studio users, as each time you regenerate a project ( if you need to include another addon ) you lose any other files and folders added and unlike other IDEs it is very tedious to add back in any shared code folders each time.

Before I start working on the frontend changes I just wanted to get some thoughts on the best way to cache/store the added source paths so they can be loaded back in by the frontend.

Right now we use addons.make to keep track of the addons added to a project.
I was hoping to avoid using another file to keep track of the file paths.

config.make has PROJECT_EXTERNAL_SOURCE_PATHS which has the same idea / functionality.
Would that make sense or be weird?

With any PG changes we might be thinking about is there a need for a file to store additional info for the frontend at a per project level? Or is config.make the best solutions for this?

Would love any thoughts @arturoc @thomasgeissl

ps: I realize @thomasgeissl #245 uses the -s argument in a different way. Happy to coordinate so we don't end up using the same args. :)

[ofTheo]

PG Frontend changes in too.
Tested on Visual Studio and Xcode.
Seems to work well for both absolute file paths and relative file paths.

[ofTheo]

going to merge this. we've been using it now for a while on mac and pc and its been a huge help ( especially for PC ).