Capture image while stream is active/inactive.

[mcelhennyi]

I would like to be able to capture still frames while streaming or while not streaming.

[mcelhennyi]

Possible to expedite this feature @shogo4405 ?

[shogo4405]

HaishinKit is an OSS project. I'm waiting your PR! Thank you.

[mcelhennyi]

Would you be able to explain, to a novice iOS developer like myself, the right way to implement this feature? I am unfamiliar with iOS and your code base. So any suggestions or guidance is welcome.