Add custom environment option

[xiaopeng12138]

use AssetBundle to make custom environment possible

WIP

[xiaopeng12138]

added in d8e8fa4

quick tutorial for adding custom environment to wacvr:

• clone wacvr's repo
• make your environment (currently you can not edit anything else that the "Room")
• make your environment to prefab
• mark it as AssetBundle
• go to Asset tap, find Make AssetBundles stuff, click it
• after that, go to Asset/AssetBundle and add ".abf" at the end of the file name
• put it into StreamingAssets/Environments folder
• Choose it on the config panel under Graphic tap

[xiaopeng12138]

Current issues:

• the original environment still stay at there
• no light map for AssetBundle

[xiaopeng12138]

Fixed light map issue (sort of