Consolidates slicing off first line of section

Section#initialize slices off first line, conditional on it starting
with a header like #SECTION_NAME. Slicing the first line also
increments @current_line by one.

AreaHeader is exempt on account of AreaHeader::is_just_one_line?
returning true.

AreaData and Helps decrement @current_line by one before going into
an each_line loop in order to compensate for the line number being
incremented before any parsing is done. (The loop body can be
skipped early, so it's easier to do it as the first thing in the
block of code and not worry about it later.)