Make app.use(koaHelmet()) optional

[ralexandr]

Hi! Could you please give the ability to turn off koaHelmet inside app.js (througn config instance) ?
I need to run reactql bundle in an environment where headers are read-only and koaHelmet can't work properly in such situation.
I will be very handy if there will be a method (smth like config.disableKoaHelmet) for managing koaHelmet (like it is already done for bodyParser).

[leebenson]

good idea - I'll add that to the next release.