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.
you can leave this as one line... :)