A consistent case pattern

[tecosaur]

I find that often I need to remember not just a colour scheme, but also the naming scheme it uses. Currently, ColorSchemes.jl has a mix of:

• nocase (e.g. twelvebitrainbow, redgreensplit, gnuplot2)
• snake_case (e.g. tableau_superfishel_stone, seaborn_dark6)
• Camel_Snake (e.g. Spectral_10)
• CamelCase (e.g. Zissou1Continuous)

I think it would be rather good if the format could be normalised, so there's less that needs to be remembered. Aliases for backward-compatibility can be easily added to the colorschemes dict.