update latest dependency

	github.com/PuerkitoBio/goquery v1.8.1
	github.com/mattn/godown v0.0.1
	github.com/microcosm-cc/bluemonday v1.0.26
	github.com/spf13/cast v1.5.1

fix docs

add sync.Map for thread safety

add size() function

Update Built-in functions:

1. remove selection function prefix "node".
2. add absHref/first/last/parents/prarentsUntil functions.

support nested functions

1. rewrite core parser
2. add example and godoc

change tager to private

fix bultin function bug

1.Priority order: struct method -> global
2.Support sub struct call parent function.
3.Change builtin function.