Be able to set custom keyword for error messages

Hi!

Thanks for your great libraries. It's cool to work with.

Being able to define the key for the error messages could be nice.

maybe something like:

```
import Ajv from 'ajv'
import ajvErrors from 'ajv-errors'

let ajv = new Ajv({ allErrors: true })
ajv = ajvErrors(ajv, { keyword: "myCustomErrorMessage" })
```

what is your opinion about that?

thank for all!