Support speech context

[bruceliu2008]

Hi, May I know will this library support speech context? if not, any work around?

Thanks, Bruce

[peterk]

Currently the library does not support speech context but it would be great if it did. Speech context helps Google Speech recognition in determining common phrases and words. For people subtitling audio from a conference event it would likely improve quality a lot.

[BingLingGroup]

api的使用与来源参见 #111 ，这并不是cloud speech api，所以不支持环境识别。