Allow exporting a bleve index to []byte

[peterwilliams97]

I need to serialize/deserialize bleve indexes to/from []byte. This PR is the best way I can see how to do so based on my reading of your code. If there is a better way, please tell me about it and ignore this PR.

The calling code that uses the proposed code change in this PR looks like this.

import (
      btreap "github.com/blevesearch/bleve/index/store/gtreap"
     "github.com/blevesearch/blevex/preload"
     "github.com/blevesearch/bleve"
)

// ExportBleveMem serializes `index` to a byte slice.
func ExportBleveMem(index bleve.Index) ([]byte, error) {
	var b bytes.Buffer
	w := &b
	i, _, err := index.Advanced()
	if err != nil {
		return nil, err
	}
	if err = preload.ExportBleve(i, w); err != nil {
		return nil, err
	}
	return b.Bytes(), nil
}

// ImportBleveMem deserializes `data` to a byte bleve.Index.
func ImportBleveMem(data []byte) (bleve.Index, error) {
	return bleve.NewUsing(
		"",
		bleve.NewIndexMapping(),
		bleve.Config.DefaultIndexType,
		preload.Name,
		map[string]interface{}{
			"kvStoreName_actual": btreap.Name,
			"preloadmem":         data,
		})
}

The code that uses this is in https://github.com/PaperCutSoftware/pdfsearch