cmd/goimports, imports: add -local flag

This allows the caller to indicate they want certain
import paths to sort into another group after 3rd-party
imports when added by goimports. For example, running
'goimports -local example.com/' might produce

    import (
        "database/sql"
        "io"
        "strconv"

        "golang.org/x/net/context"

        "example.com/foo/bar"
        "example.com/foo/baz"
    )

Resolves golang/go#12420

Change-Id: If6d88599f6cca2f102313bce95ba6ac46ffec1fe
Reviewed-on: https://go-review.googlesource.com/25145
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
3 files changed