app_read: Add an option to return terminator on empty digits.

app_read: Add an option to return terminator on empty digits.

Adds 'e' option to allow Read() to return the terminator as the
dialed digits in the case where only the terminator is entered.

ie; if "#" is entered, return "#" if the 'e' option is set and ""
if it is not.

ASTERISK-30411

Change-Id: I49f3221824330a193a20c660f99da0f1fc2cbbc5