Add support for SymbolNode

- Add tests for SymbolNode;
- Change tests for SymbolNode, WhiteSpaceNode, and
  PunctuationNode sub-classing from Text instead of Parent.

Related: https://github.com/wooorm/nlcst/issues/1 and
https://github.com/wooorm/nlcst/issues/2
Update changelog for 0.2.0
0.2.0