Allow named bindings to be escaped.

[trevorsmith]

This PR extends escaping to named parameter bindings.

knex.select('*').from('users').whereRaw(":property: = '\\:value' OR \\:property: = :value", {
  property: 'name',
  value: 'Bob'
})

Outputs:

select * from `users` where `name` = ':value' OR :property: = 'Bob'

I've included tests and updated the documentation as well.

[elhigu]

Looks good to me 👍 Thanks