Add ability to add named imports in a separate group

[ustun-ed]

We have the following internal code style:

• Standard library imports
• repository imports
• Vendor imports
• Named imports (all internal and external named should be in same group)

Example:

import (
	"context"
	"fmt"
	"time"

	"github.com/ourcompany/ourcompany/pkg/log"
	"github.com/ourcompany/ourcompany/pkg/percentile"

        "github.com/google/go-cmp/cmp"

	svc "github.com/ourcompany/ourcompany/pkg/aggregator_service"
)

Could you add support for the final named imports group?

[ustun-ed]

Added a draft PR above.