feat: add custom field types to open modal insert

[nzmattman]

I have added an option for creating custom HTML form types by using a call back function as the type.

• Created the callback function
• Adjusted the HTML build process to check for the function type and inject that HTML, otherwise use the standard HTML markup
• adjusted the validation rules to target new markup

I have also adjusted the modal css to allow for this integration to work better;

• changed to use flexbox over absolute positioning
• added styling support for select lists and textareas (rather than just input fields)

[Alex-D]

This idea is brilliant! :)

But, I can't merge this since it's full of breaking changes :(

Trumbowyg 2 partially supports IE11, with this, it will completely break the support of all old browsers, which means that I need to push that in a new major version...

Can we make that work without flexbox, and without ES2015+ things?

[nzmattman]

Ahh right, bloody IE!

Yip, I will change it to not use flexbox. I haven't had to code anything for IE11 for a long time, but will do

[nzmattman]

Ok, I have done a few updates for this.

I am not really happy with how the example in the docs looks, but the general code is there

[Alex-D]

I will check that in old browsers in the next few days, thank you for your reactivity! :)

[Alex-D]

Thank you for this PR :)

[Alex-D]

I've fixed some bugs on the develop branch directly :)

[nzmattman]

You are a champion, thanks Alex.
I didn't have time on Friday to look at this and was going to fix up the small things today.