ability to specify Joi validation options without using Joi objects as schemas
Hey there!

Considering using koa-joi-router on our next few projects.

One issue I ran into is no ability to set options for Joi other than specifying the validation as Joi objects everytime and chaining an options call.
`body: joi.object({
  email: joi.string().email().required(),
  password: joi.string().required(),
}).options({abortEarly: false}),`

This is a bit tedious if you want something set for every validation everywhere.
F.ex. I'd like abortEarly: false to get all validation errors at once.

The way I see it, it could be done 2 ways:
1. have individual options in the validation spec (abortEarly, convert, etc.)
2. whole options object in the validation spec

Potential problems:
These "global" options overriding individual validations sent as Joi objects with options specified (like in the example above).

I'd be willing to tackle this and submit a PR if everyone's ok with the changes - already have solution 1. ready.