Introduce `evil-leader/no-prefix-mode-rx'.

Addresses issue #16.

Signed-off-by: Michael Markert <markert.michael@googlemail.com>
Document `evil-leader/no-prefix-mode-rx' in the FAQ.

Signed-off-by: Michael Markert <markert.michael@googlemail.com>
Clarify docstring.

Signed-off-by: Michael Markert <markert.michael@googlemail.com>