Increase separation of concerns and configurability of parsing

#### Support plan

* *is this issue currently blocking your project?* (yes/no): no
* *is this issue affecting a production system?* (yes/no): no

#### Context

* *node version*: na.
* *module version*: `7.0.3`
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): hapi
* *any other relevant information*:

#### What problem are you trying to solve?

I want to be able to add [custom parsing](https://github.com/hapijs/hapi/issues/4223) capabilities to my Hapi server.

While investigating about how this could be achieved, I came across the [hard coded list of parsers](https://github.com/hapijs/subtext/blob/c59fcd3e085dea3ee021bd0ad0d55cfbf16b26e3/lib/index.js#L175-L216) in this module.

It would be very convenient to be able to be able to pass the list of parsers as an option. IMHO the default list of parsers should not even be coded in this module at all (though I understand this would be a big breaking change).

#### Do you have a new or modified API suggestion to solve the problem?

Be able to provide a custom list of parsers in the options. The default parsers would look like:
```js
[
// Binary

{
mime: 'application/octet-stream',
parse: (payload) => (payload.length ? payload : null),
},

// Text

{
mime: /^text\/.+$/,
parse: (payload) => payload.toString('utf8'),
},

// JSON

{
mime: /^application\/(?:.+\+)?json$/,
parse: (payload, mime, options) => {
if (!payload.length) {
return null
}

try {
return Bourne.parse(payload.toString('utf8'), { protoAction: options.protoAction })
} catch (err) {
const error = Boom.badRequest('Invalid request payload JSON format', err)
error.raw = payload
throw error
}
},
},

// Form-encoded

{
mime: 'application/x-www-form-urlencoded',
parse: (payload, mime, options) => {
const parse = options.querystring || Querystring.parse
return payload.length ? parse(payload.toString('utf8')) : {}
},
},
]
```