Contao extension to extend form fields with more options.

• Adds a minimum options and maximum options setting to checkbox and multiple select fields.
• Allows you to define a custom error message in the form field's options.
• Adds the ability to provide a custom input field in the front end for radio button form fields.
• Allows you to define disallowed words for text and textarea fields.
• Allows you to define allowed words for text fields.
• A httpurl rgxp option (see this PR).
• Allows you to show certain form fields only to specific member groups.
• Allows you to show a character count for textareas in the front end.
• Allows you to load a default value from a query parameter of the request.
• Automatically uses the native HTML5 date input type for date text fields.
• Allows you to automatically add a blank option to <select> fields.