Add gflag support
Add gflag support

Summary:
This diff allow user to pass basic (bool/int/string) gflag and convert to StargateOptions.

Need to add support for vector of int/string, plan to use "xxx,yyy,zzz" to format
Hacked on some macro. ldemailly

Reviewed By: ldemailly

Differential Revision: D4191042

fbshipit-source-id: ab17d7ea8cae4ad8a6193e60c59a6da210e883c5

luluandleilei/wdt,luluandleilei/wdt,ldemailly/wdt,luluandleilei/wdt,ldemailly/wdt,luluandleilei/wdt,ldemailly/wdt,ldemailly/wdt