"C-x C-f" can un-ignore dotfiles with a leading dot input

"C-x C-f" can un-ignore dotfiles with a leading dot input

* counsel.el (counsel-find-file-ignore-regexp): Add a recommended value,
all dotfiles.
(counsel--find-file-matcher): When `ivy-text' starts with a dot, don't
use `counsel-find-file-ignore-regexp'. The generic way to do this is
with "C-c C-a" (`ivy-toggle-ignore'), but this is faster and more convenient.

Re #408