Add uet run command

[hach-que]

This adds a uet run command that runs the editor, game or related tool such as UAT or UBT. This is primarily useful so you don't have to remember the exact path to tools such as UAT.

The 'target' argument specifies what to run, and it can be any of the following:

editor:         Run the Unreal Engine editor.
editor-cmd:     Run the Unreal Engine editor as a command-line application.
game:           Run the Unreal Engine game for this project on the current platform. This uses 
                the editor binary to quickly launch the game. This can only be used in a project context.
uat:            Run UnrealAutomationTool.
ubt:            Run UnrealBuildTool.
uba-visualizer: Run the Unreal Build Accelerator visualizer.

If --path points to a project file, the target will automatically receive the project file as an argument in an appropriate manner, if possible.

This command uses the same resolution logic as uet build and therefore has sensible heuristics - running it in a directory with a .uproject file will pick the engine version based on what's in the .uproject file, running it in an engine directory will automatically use that engine, so on and so forth.

uet uat will not automatically supply the -project argument (as it is not permitted unless the UAT sub-command allows it). To have the -project argument automatically supplied to UAT, use uet uat --project <args> instead.

If you don't specify a target on the command line, it defaults to the running the editor.