A composed character is a way of visually replacing a string of characters with
a single glyph. If beg in the call to evil-range lies in this hidden sequence,
adjust the position to the beginning of the sequence. Similarly, make sure end
comes at the end of any such hidden sequence. The purpose is to prevent evil
from unintentionally modifying part of a hidden sequence of characters.

Account for beg > end case and case where beg or end are nil

Allows user to disable special handling of composed characters. See the
docstring for more information.