Allow launching directly into games w/ dev cheats

Usage is simple; just pass Launch.Developer=true into the command line
arguments.

Launching can further be customized with:

  Launch.Developer.Map: Select the map to launch
  Launch.Developer.Cheats: Specify which cheats to enable
  Launch.Developer.Bots: Specify which bots to use. This also limits the
    maximum number of bots that will be used in the map. For example, to
    use no bots specify Launch.Developer.Bots=""

A reasonable default is selected for each Launch.Developer.* argument. if
Launch.Developer=true is not present, then OpenRA will start normally.

An example launch is as follows:

OpenRA.Game.exe \
  Launch.Developer=true \
  Launch.Developer.Map=texas-sea \
  Launch.Developer.Cheats="DevShroudDisable, DevFastBuild" \
  Launch.Developer.Bots="Rush AI, Turtle AI"

This will launch a game directly into the texas-sea map with the shroud
disabled and fast build enabled. Further, there will be two AI opponents,
one using the Rush AI and the other using the Turtle AI.