feature request: support key for ButtonGlow

[lwalter]

Hello,

Thanks for the very useful library. In some lua code I am writing for an addon I need to leverage the key feature of various glows as this ensures multiple glows can be start/stopped in isolation of each other. All of the glow types support this aside from ButtonGlow. Would it be possible to support passing in a key to ButtonGlow_Start and ButtonGlow_Stop similar to the other start/stop methods?

Thanks!