encodePNG implemented

Fixes #2.

Allows passing in an io.Writer to save a PNG.

context.go

@@ -5,6 +5,8 @@ import (
 	"image"
 	"image/color"
 	"image/draw"
+	"image/png"
+	"io"
 	"math"
 
 	"github.com/golang/freetype/raster"
@@ -103,6 +105,11 @@ func (dc *Context) SavePNG(path string) error {
 	return SavePNG(path, dc.im)
 }
 
+// EncodePNG encodes the image as a PNG and writes it to the provided io.Writer.
+func (dc *Context) EncodePNG(w io.Writer) error {
+	return png.Encode(w, dc.im)
+}
+
 // SetDash sets the current dash pattern to use. Call with zero arguments to
 // disable dashes. The values specify the lengths of each dash, with
 // alternating on and off lengths.