v0.3.0

• Add viridis, inferno, magma and plasma sequential color scales.
• Quantitative scales now return ticks in descending order for descending domains.
• Non-linear quantitative scales are now more accurate.
• Clamped scales can no longer return a value slightly outside of the domain when inverting.
• log.ticks now filters powers-of-base ticks for very large domains.
• log.tickFormat is now more accurate.
• Quantize scales are now more accurate.
• quantize.invertExtent is now consistent with quantize.