added icon, shorten and whiten label

[namgk]

Hi there,

Thanks for the node.

I think it will look nicer with a white label and an icon so I make this pull request to beautify the appearance a little bit. Also I think the label could be shortened without the "writer" so that it only takes 2 lines in the palette.

Thanks

[chughts]

@namgk Thanks for the pull request. Could you also please sign the CLA - https://cla-assistant.io/ibm-early-programs/node-red-contrib-facebook-messenger-writer?pullRequest=3

[namgk]

all done, thanks.