derive a dom selector lib

[leeoniya]

DropCSS already contains the vast majority of the machinery necessary to create something akin to these libs:

https://github.com/fb55/css-select
https://github.com/jquery/sizzle
https://github.com/dperini/nwmatcher

the amount of additional code/tweaks should be quite small to get this working and there's a reasonable chance that a selector engine built using DropCSS's innards would be very competative in speed and definitely size. Sizzle is 19.4KB (min) while DropCSS is 9.55KB (min) which includes an HTML parser [1] and post-processor [2] that can mostly be ditched.

[1] https://github.com/leeoniya/dropcss/blob/master/src/html.js
[2] https://github.com/leeoniya/dropcss/blob/master/src/postproc.js