Check for invalid field and add CSS hook

Currently, you can't setup a field using `[dynamictext*]` and use the same CSS hooks you'd use for normal `[text*]` field validations.  Typically there's a class of `wpcf7-not-valid` added to `[text*]` fields in CF7 on the frontend.  I've added a function that checks for an error and if there is one, appends that class to the HTML output for that field.

I've also added a base CSS class to the output element so that, by default it has classes of `wpcf7-text` and `wpcf7-form-control` because I believe that is what CF7 outputs by default.   I realize we might be able to use a filter but I constantly run into this issue when setting up websites using this plugin and since CF7 uses that as a default class, maybe this plugin should too.  I use some JS that looks for some CF7 fields and it's easier to use `$('.wpcf7-form-control')` if I want to target all form elements including the dynamic text fields.