Allow to run non-Development Editor + few minor fixes in the same code

[sleeptightAnsiC]

Hi @adamrehn ,

I fixed the suport for launching non-Development Editor #56 There are also few other changes and fixes added by this PR, as they were either easy to fix, located in the same code or blocking me from fixing the mentioned issue. I tested it mostly on Linux but it should work™ on other platforms too since the underlying code was already there.

I also tried to deal with #61 and #62 but they don't seem worth fixing right now (see the comments under those issues). Though, some changes were made with those in mind.

Let me know what do you think. PR is open for your edits.

Cheers!

All changes:

• API now allow to run non-Development Editor
• You can do stuff like ue4 run DebugGame and ue4 editor DebugGame as both these commands now take [CONFIGURATION] as optional parameter
• Fix: "ue4 run DebugGame" runs default Development executable instead of DebugGame #56
• add validBuildTargets helper function that behaves similar as validBuildConfigurations
• Fix: add validation for build target inside of getBuildScript as this was not handled anywhere unlike build configuration
• Fix: validBuildTargets/Configurations now take into consideration if UE is installed via EpicLauncher or not (now the behavior lines up with the tables on this page)
• allow to build Game target inside of getBuildScript as this was fairly easy to add was surprisingly not supported
• added small FIXME note that we are missing support for 'Program' build target