make `this._ast` public as `this.ast`
make `this._lines` public as `this.lines`
Revert "make `this._lines` public as `this.lines`"

This reverts commit 55898c8ff8eb2202fa934540127b9d1972564cd0.
add more helpers
make `this._lines` public as `this.lines`
improve tokenizations for better customizations
clean code
improve directive @include

Three directives @include, header, footer, can start at any column now.