Sourcemap annotations should be string, not boolean, values

[Bendihossan]

• See postcss/autoprefixer docs: https://github.com/postcss/autoprefixer/blob/master/README.md#source-map
• Adds test to assert a string annotation is handled correctly

I came across this because in my project's Gruntfile.js I use LESS to generate a sourcemap and add an annotation to the output, then run autoprefixer however this meant it would generate a new sourcemap based on the single .css output from LESS and so the sourcemap didn't actually map the LESS files used to build it.

With this change the the annotation option will take a string which will specify what the annotation comment value should be, which the autoprefixer then updates the outputted CSS with.

[Bendihossan]

Thanks for merging & tagging @nDmitry :)