Add SetSheetViewOptions and GetSheetViewOptions (#145)

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.

1 files changed, 11 insertions, 0 deletions

diff --git a/lib.go b/lib.go
index b28b50f..2f3df35 100644
--- a/lib.go
+++ b/lib.go
@@ -121,3 +121,14 @@ func deepCopy(dst, src interface{}) error {
 	}
 	return gob.NewDecoder(bytes.NewBuffer(buf.Bytes())).Decode(dst)
 }
+
+// boolPtr returns a pointer to a bool with the given value.
+func boolPtr(b bool) *bool { return &b }
+
+// defaultTrue returns true if b is nil, or the pointed value.
+func defaultTrue(b *bool) bool {
+	if b == nil {
+		return true
+	}
+	return *b
+}