Added before_parse_content plugin trigger

The `before_parse_content` is useful to modify the Markdown content before it is converted to HTML.

In some cases this can be done using `after_load_content` trigger. However that trigger is called before loading the config and meta, making it impossible to modify the content based on any settings.

Consistently the `content_parsed` trigger is renamed to `after_parse_content`. To retain B.C. the `after_parse_content` method calls `content_parsed` by default.