add exact_by_default=true to .flowconfig (#3980)
add exact_by_default=true to .flowconfig (#3980)

Summary:
matching `.flowconfig` option used in dev env setups internally at Meta where we sync relay into

Pull Request resolved: https://github.com/facebook/relay/pull/3980

Reviewed By: captbaritone

Differential Revision: D37246522

Pulled By: voideanvalue

fbshipit-source-id: 41e4052e0c14c660fd9c3116a2eb68a928716120

facebook/relay,voideanvalue/relay,xuorig/relay,voideanvalue/relay,facebook/relay,xuorig/relay,facebook/relay,xuorig/relay,voideanvalue/relay,voideanvalue/relay,xuorig/relay,facebook/relay,facebook/relay,voideanvalue/relay,xuorig/relay,voideanvalue/relay,xuorig/relay,facebook/relay