Added view configuration to SheetView

Hi @xuri 

I'm adding an additional configuration, for how sheet is presented:

Normal:
<sheetViews>
    <sheetView workbookViewId="0" tabSelected="1"/>
</sheetViews>

Page Break Preview: `view="pageBreakPreview" `

```
<sheetViews>
    <sheetView workbookViewId="0" zoomScaleNormal="100" zoomScale="60" view="pageBreakPreview" tabSelected="1"/>
</sheetViews>
```

Page Layout: `view="pageLayout"`

```
<sheetViews>
    <sheetView workbookViewId="0" zoomScaleNormal="100" view="pageLayout" tabSelected="1"/>
</sheetViews>
```

Img reference from stackoverflow: https://i.stack.imgur.com/zxlGQ.png

Author: JDavidVR

sheetview.go

@@ -59,6 +59,11 @@ type (
 	// when the flag is true. (Default setting is true.)
 	ShowZeros bool
 
+	// View is a SheetViewOption. It specifies a flag indicating
+	// how sheet is displayed, by default it uses empty string
+	// avl options: pageLayout, pageBreakPreview
+	View string
+	
 	/* TODO
 	// ShowWhiteSpace is a SheetViewOption. It specifies a flag indicating
 	// whether page layout view shall display margins. False means do not display
@@ -80,6 +85,14 @@ func (o *TopLeftCell) getSheetViewOption(view *xlsxSheetView) {
 	*o = TopLeftCell(string(view.TopLeftCell))
 }
 
+func (o View) setSheetViewOption(view *xlsxSheetView) {
+	view.View = string(o)
+}
+
+func (o *View) getSheetViewOption(view *xlsxSheetView) {
+	*o = View(string(view.View))
+}
+
 func (o DefaultGridColor) setSheetViewOption(view *xlsxSheetView) {
 	view.DefaultGridColor = boolPtr(bool(o))
 }