Add option for colorful toolbar button

[adam-p]

With the new logo we're switching to monochrome buttons on all platforms. Some people asked for this, but it might not actually be a great idea, might be contentious, and might actually be incorrect (the Chrome browserAction documentation explicitly says to not do that).

+1 this issue if you would prefer a colourful button. (Not just if you want the option -- only if you actually want and would use the colourful version. I only have so much time.)

[Hamdiakoguz]

+1

[dalin-]

-1

The chrome should be monochrome and not steal attention away from the actual content.

[rwagner00]

I love the monochrome button for normal interaction, but personally would prefer if the button lit up when Markdown had been toggled on. That might be more trouble than you want to go to, but it's a thought.

[adam-p]

@rwagner00: That's a different issue: #258