Reworked JS helpers with APi #17

[kirs]

@dfyx noticed that there is no way to add new fields in JS without using buttons.
Here is my implementation (may not work yet on edge cases) with an API:

$(".task_list").addFormField("tasks") # receives association name, .task_list is container with fields
$(".task_list").removeFormField("tasks", 2) // destroys second field in the form

Example of form code can be found here: https://github.com/kirs/activeform/blob/e9375a9d6fac5e0e9ff893dc30e22e2e8bb112dd/test/dummy/app/views/assignments/_form.html.erb

[kirs]

Probably needs some client-side code review. Are we OK with shipping the code as jQuery plugin?

[m-Peter]

Well done @kirs ! 👍

[kaspth]

Won't inserting a script element break turbolinks?

[kirs]

My first idea was to create a hidden div with content, but as my colleague told script is better than div because it's not rendering in DOM.
We can also use the template tag from HTML5 instead of script.

[kirs]

Done. If we're ok with implementation, let's merge it. @m-Peter @kaspth @guilleiguaran