update README.
add support of attribute
Add support of adjecent selector.
- Remove child selector, replace it with Combinator ( '+', '>')
Refactor: correctly name the push operation of selectors
Add support for dash (|=) operator in attribute
Support first-child pseudo class
correct type of universalOrTypeSelector
add last-child
fix type warnings
add support for pseudo class: last-of-type and first-of-type
Refactor: use a map to map the pseudo class
add support of pseudo class empty
add nth-child to gramma