Add plugins option

[fartinmartin]

A request to add a plugins array option for users to manipulate the parsed text and transforms after style-parser has parsed markdown, but before it has returned the TextStyle object.

Each plugin would have access to a couple of utility functions: updateTransforms and getStylesAtIndex to make some generic actions easier to manage.

For some simple/contrived examples of a plugin, check out the index.test.ts file:

• simple plugin
• more complex plugin

For a more complex plugin, see the use case that inspired the plugin API request: style-parser-plugin-wrap