Feature: ability to provide custom imports sorter function

[rzajac]

Give user ability to provide custom imports sorter function.

https://github.com/dave/jennifer/blob/be82eb6738040b1b5934f0c463273c37d5fd5fd9/jen/jen.go#L132

Also it would be nice to have ability to put empty lines to achieve imports like this:

import (
	"errors"
	"fmt"
	"testing"

	"github.com/rs/zerolog"
	"github.com/rzajac/zrr/zrrtest"
	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/mock"
	"github.com/stretchr/testify/require"
	"github.com/stretchr/testify/suite"
)