Add strikethrough support

[tarrouye]

Goals :soccer:

• Add strikethrough element that matches for markdown strikethrough (~~example~~ ~~example~~)
• Add underline style/color styling options to all elements

Implementation Details :construction:

• Try to match the existing architecture as closely as possible
• Use the underline styling option to further differentiate links in default parser
  • This change can be undone and the underline can be left simply as option if you prefer to leave it out of the default parser for this library.

Testing Details :mag:

• No changes