use icons in help_text by setting help_text_icon in helper.layout.field

I tryed to use crispy in my project, where I use bootstrap and font-awesome

I found that it is very difficult to add icons (`<i class="icon-foo"></i>`) to different parts of the form.
Still in many cases it is possible without using `HTML` layout or rewriting templates.
But when I needed to add an icon to `help_text` I ran into troubles: I had to rewrite `bootstrap/layout/help_text.html` hard-coding desired icon there. (I can't touch `forms.py`)
I don't know now to add second field with different help text icon without rewriting almost all `bootstrap/*` templates.

With this branch one can do it easy:

    layout.Field(
        'fieldname',
        help_text_icon='info-sign',
    ),

This is just sort of prove of  concept, because I don't know crispy internals good enough...