vi-mode zle: how to delete past the start char of insert mode (like VIM 'backspace'=start option)

[Anton Sharonov <anton.sharonov@xxxxxxxxx>]

Hi group.

Background: I am middle-experienced VIM+bash user, and now I'm
evaluating zsh.

Question:
in my ~/.zshrc I have vi-mode command line editing enabled:

  bindkey -e

After I type some text, leave insert mode, and enter it again,
and try to delete the text I entered, for instance:

  ls ~/bin<ESC>a<C-W><BS><C-W><C-U><BS><C-W><C-U><BS><BS>...

I got cursor only till the point, where insert mode is started
(in this example right after the "~/bin").

Desired behaviour: <C-W>, <BS>, <C-U> should remove the WORD,
symbol, to the begin of line, respectively. (In VIM the
corresponding option is 'backspace', value "start" controls this
behaviour)

How can I tell zshzle that I like to be able to delete past the
point of _start_ of the insert mode ?

-- 
Anton