improve logging of visited nodes
improve logging of visited nodes

When logging visted nodes for debugging purposes, indent them
based on the current recursion level of visit().  This helps
keep clear which piece of code is responsible for which outputs.

Output (with the logging uncommented) looks like:

node: SourceFile
| node: SyntaxList
| | node: VariableStatement
| | | node: VariableDeclarationList
| | | | node: VarKeyword
| | | | node: SyntaxList
| | | | | node: VariableDeclaration
| | | | | | node: StringKeyword
| | | | | | node: Identifier
| | | | | | node: ColonToken
| | | | | | node: StringKeyword
| | | node: SemicolonToken
| | node: VariableStatement
[...]

drjanitor/tsickle,lucidsoftware/tsickle,angular/tsickle,drjanitor/tsickle,alexeagle/tsickle,alexeagle/tsickle,ncfxy/tsickle,angular/tsickle,drjanitor/tsickle,drjanitor/tsickle,lucidsoftware/tsickle,lucidsoftware/tsickle,ncfxy/tsickle,hess-g/tsickle,ncfxy/tsickle,hess-g/tsickle,alexeagle/tsickle