Feature: add ability to completely clean embedded binary content at run time

[wtask]

Use case: the go-bindata is used to embed text/template and html/template sources which require to "compile" them (Parse) before use.
After parsing there are source and parsed template which increase used RAM.
Will be useful to have something like cleanAsset(name string) to drop any data of underlying var and remove it key from _bindata map.

[Noofbiz]

Could be nice for games too, removing textures once they're uploaded to the gpu