Two new methods:
- SetSheetViewOptions(sheetName string, viewIndex int, opts ...SheetViewOption) error
- GetSheetViewOptions(sheetName string, viewIndex int, opts ...SheetViewOptionPtr) error

The option values are given by the user through types that have privates methods
that implement the private SheetViewOption and SheetViewOptionPtr interfaces:
- DefaultGridColor(bool)
- RightToLeft(bool)
- ShowFormulas(bool)
- ShowGridLines(bool)
- ShowRowColHeaders(bool)

Examples:
    err := xl.SetSheetViewOptions("Sheet1", -1, excelize.ShowGridLines(true))

    var showGridLines excelize.ShowGridLines
    err := xl.GetSheetViewOptions("Sheet1", -1, &showGridLines)

Fixes #145.