Add NoSync field to Options

This allows enabling NoSync when you only have access to the Options
passed into Open() and not the returned DB (as is frequently the case
with libraries).

db.go

@@ -160,6 +160,7 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 	if options == nil {
 		options = DefaultOptions
 	}
+	db.NoSync = options.NoSync
 	db.NoGrowSync = options.NoGrowSync
 	db.MmapFlags = options.MmapFlags
 	db.NoFreelistSync = options.NoFreelistSync
@@ -1008,6 +1009,11 @@ type Options struct {
 
 	// PageSize overrides the default OS page size.
 	PageSize int
+
+	// NoSync sets the initial value of DB.NoSync. Normally this can just be
+	// set directly on the DB itself when returned from Open(), but this option
+	// is useful in APIs which expose Options but not the underlying DB.
+	NoSync bool
 }
 
 // DefaultOptions represent the options used if nil options are passed into Open().