Handling escape symbols

From https://acss.io/#pseudo-classes:

Each line should parse as one word (i.e. one identifier):

C\(\#0280ae\)
C\(brandColor\)
C\(\#fff\)\:h:hover

This can be verified with https://rawgit.com/tabatkins/parse-css/master/example.html

This issue continues from https://github.com/shellscape/postcss-values-parser/issues/93

With thanks and credit to @nex3 and @ai for identifying this issue.

Update: I have started work on updating the Tokenizer, but I may need assistance as I integrate or abandon the current multichar tokens. I don’t necessarily see how those tokens benefit speed. Any risk of inaccuracy seems too steep a price to pay.

@ai, thank you for the wonderful documentation @ https://github.com/postcss/postcss/blob/7.0.27/docs/architecture.md#tokenizer--libtokenizees6-