Factor out JSON stripComments().
Factor out JSON stripComments().

Summary:
Comments are a useful extension to JSON, especially for configuration files.

Test Plan: fbconfig common/json mcrouter/lib/config tupperware/agent && fbmake runtests

Reviewed By: aravindn@fb.com

Subscribers: anarayanan, pavlo, stepan, dipanshu, alikhtarov

FB internal diff: D1493963

Tasks: 4686066

zhlong73/mcrouter,apinski-cavium/mcrouter,reddit/mcrouter,synecdoche/mcrouter,easyfmxu/mcrouter,glensc/mcrouter,nvaller/mcrouter,is00hcw/mcrouter,facebook/mcrouter,synecdoche/mcrouter,zhlong73/mcrouter,seem-sky/mcrouter,yqzhang/mcrouter,evertrue/mcrouter,leitao/mcrouter,nvaller/mcrouter,is00hcw/mcrouter,seem-sky/mcrouter,evertrue/mcrouter,reddit/mcrouter,apinski-cavium/mcrouter,facebook/mcrouter,reddit/mcrouter,leitao/mcrouter,nvaller/mcrouter,seem-sky/mcrouter,facebook/mcrouter,seem-sky/mcrouter,yqzhang/mcrouter,nvaller/mcrouter,reddit/mcrouter,evertrue/mcrouter,is00hcw/mcrouter,tempbottle/mcrouter,facebook/mcrouter,tempbottle/mcrouter,synecdoche/mcrouter,zhlong73/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,tempbottle/mcrouter,easyfmxu/mcrouter,glensc/mcrouter,tempbottle/mcrouter,glensc/mcrouter,leitao/mcrouter,zhlong73/mcrouter,yqzhang/mcrouter,evertrue/mcrouter,is00hcw/mcrouter,leitao/mcrouter,glensc/mcrouter,apinski-cavium/mcrouter,apinski-cavium/mcrouter,easyfmxu/mcrouter,easyfmxu/mcrouter