Add support for launching directly into games w/ dev cheats enabled

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.