color: expose SetWriter and UnsetWriter

For some users performance is very important and hence having access to
the underlying writer functions allow them to use bytes buffer and avoid
string allocations.

closes #124

color.go

@@ -163,7 +163,10 @@ func (c *Color) unset() {
 	Unset()
 }
 
-func (c *Color) setWriter(w io.Writer) *Color {
+// SetWriter is used to set the SGR sequence with the given io.Writer. This is
+// a low-level function, and users should use the higher-level functions, such
+// as color.Fprint, color.Print, etc.
+func (c *Color) SetWriter(w io.Writer) *Color {
 	if c.isNoColorSet() {
 		return c
 	}
@@ -172,7 +175,9 @@ func (c *Color) setWriter(w io.Writer) *Color {
 	return c
 }
 
-func (c *Color) unsetWriter(w io.Writer) {
+// UnsetWriter resets all escape attributes and clears the output with the give
+// io.Writer. Usually should be called after SetWriter().
+func (c *Color) UnsetWriter(w io.Writer) {
 	if c.isNoColorSet() {
 		return
 	}
@@ -197,8 +202,8 @@ func (c *Color) Add(value ...Attribute) *Color {
 // On Windows, users should wrap w with colorable.NewColorable() if w is of
 // type *os.File.
 func (c *Color) Fprint(w io.Writer, a ...interface{}) (n int, err error) {
-	c.setWriter(w)
-	defer c.unsetWriter(w)
+	c.SetWriter(w)
+	defer c.UnsetWriter(w)
 
 	return fmt.Fprint(w, a...)
 }
@@ -220,8 +225,8 @@ func (c *Color) Print(a ...interface{}) (n int, err error) {
 // On Windows, users should wrap w with colorable.NewColorable() if w is of
 // type *os.File.
 func (c *Color) Fprintf(w io.Writer, format string, a ...interface{}) (n int, err error) {
-	c.setWriter(w)
-	defer c.unsetWriter(w)
+	c.SetWriter(w)
+	defer c.UnsetWriter(w)
 
 	return fmt.Fprintf(w, format, a...)
 }
@@ -241,8 +246,8 @@ func (c *Color) Printf(format string, a ...interface{}) (n int, err error) {
 // On Windows, users should wrap w with colorable.NewColorable() if w is of
 // type *os.File.
 func (c *Color) Fprintln(w io.Writer, a ...interface{}) (n int, err error) {
-	c.setWriter(w)
-	defer c.unsetWriter(w)
+	c.SetWriter(w)
+	defer c.UnsetWriter(w)
 
 	return fmt.Fprintln(w, a...)
 }