Add 'negate' option to text component

[donmccurdy]

three-bmfont-text is based on an older version of msdfgen, and fonts generated with the newer version don't work. I've proposed a fix (Experience-Monks/three-bmfont-text#24) but we'd need to add the same option to our own shader.

[donmccurdy]

I made a web-based bmfont packer if you want to test this: http://msdf-bmfont.donmccurdy.com/. But the output will not work without this property.