Added support for custom.css
Update
Remove the need for a transient by moving the construction of the code for the chart in the output filter.
The resulting code will still benefit from TablePress output caching in transient, as it uses the existing transient caching mechanism.
Remove the `charist_table_hide` option and always hide the table.
This gives more flexibility as a user can simply use the default [table] Shortcode to also show the table. This also saves us from having to deal with where to place the table (before or after the chart, with intermediate content, etc.).
Change the default value for the `chartist` Shortcode parameter to false, and use easier to read logic in the output filter.
Clean up white space, coding standards, and inline documentation.
Cut off the header row in a more intuitive way.
Simplify JSON data generation by only using one if-condition for the table head row.
Use the HTML ID of the table for the unique ID of the chart HTML container, to allow multiple charts on one page.
Use a "Custom CSS" file that is stored in the `wp-content` directory of the site. Placing it inside the plugin folder will not work, as the CSS file will be deleted during a plugin update by WordPress.
Clean-ups in the readme files.