areverse: reverse the order of th input lines

Parameters:
-e {expr}: define the expression where the reversing must be restarted
-i: by defult, the line where the reversing must restart belogs to the next
  chunk/paragraph. With this option on, these lines belong to the previous
  chunk.
-bs {size}: specify the buffer size to check the previous lines in the
  restarting expression. The runtime variable is called 'lines'.

Examples:
acat ... | areverse
  -> the lines are shown in the reversed order

... | areverse -e "line []= 'apple'"
  -> reverse in chunks. When a line contains 'apple', the previous lines are
     reversed and printed, and the process restarts with the current line.

     Input:
     grape1
     grape2
     apple
     plum

     Output:
     grape2
     grape1
     plum
     apple

... | areverse -e "line []= 'apple'" -i
  -> the previous one, but here the current line is part of the first reordering
     (the last line of the prev. chcuk -> the first line on the display)

     Input: take the previous one
     Output:
     apple
     grape2
     grape1
     plum

... | areverse -bs 1 -e "lines[0] []= 'apple' && lines[1] []= 'plum'"
  -> the reordering takes only place when the current line contains 'apple' and
     the previous line contains 'plum'