Add functionality to super-class SymbolNode from Element to Text
Add tests to super-class SymbolNode from Element to Text
Add docs for SymbolNodes new super-class
Merge branch 'feature/symbol-node-sub-class-text'

This sub-classes SymbolNode from Text instead of Element.

This additionally changes the super-class chain for WhiteSpaceNode
and PunctuationNode.

Related: https://github.com/wooorm/nlcst/issues/2
Update changelog for 0.3.0-rc.2
0.3.0-rc.2