Add printBitmapBase64Custom such as printBitmapCustom supports Base64

[mitsuharu]

Hello.

Your library is very useful.

I want printBitmapCustom which can support Base64 image as well as printBitmap, so I made a PR. I don't understand Chinese, so I change only a number in the comments.

This printBitmapBase64Custom has been tested in my application. https://www.youtube.com/watch?v=yuoYod6Y_Ao

[Surile]

ok，thanks。