Support inline source maps in Sprockets

[Mr0grog]

This is somewhat related to #127, but a little different/simpler.

Autoprefixer-rails can currently read in an inline sourcemap and update it if you set the map: true option when calling it directly:

result = AutoprefixerRails.process(source_css, map: true, from: 'in.css', to: 'out.css')

…but if you’re using the Sprockets integration, it doesn’t seem like there is any way to set it:

autoprefixer-rails/lib/autoprefixer-rails/sprockets.rb

Lines 20 to 22 in 987430a

	def self.run(filename, css)
	output = filename.chomp(File.extname(filename)) + ".css"
	result = @processor.process(css, from: filename, to: output)

I think it would be really helpful if the Sprockets integration looked for an inline source map in the source CSS string and, if present, automatically set map: true when running the processor.