filter nodes

[knsh14]

some people want to ignore some node.
I think some refactor step to achieve.

1st step

change tree function.
use switch statement to call specific func to using map to get specific func.
before

switch n.(type) {
  case *ast....
}

after

f := functions[fmt.Sprintf("%T", node)]
f(w, ....)

2nd step

create empty function to pass node.

3rd step

set pass function to map when ignore node is given.

[knsh14]

need to add Ignore option to astree.Tree func. then generate map with option and execute internal tree func.

[knsh14]

https://budougumi0617.github.io/2018/09/21/package-dependencies-with-go-list-and-build-tags/