Adds --release flag to ./mach build-cef

Requested by kmc in IRC.

```
11:36 < kmc> while you're mach-ing about, can you modify build-cef to
accept --release like build does?
```
auto merge of #4077 : mttr/servo/mach_build_cef_release, r=kmcallister

Requested by kmc in IRC.

```
11:36 < kmc> while you're mach-ing about, can you modify build-cef to
accept --release like build does?
```