Program to fetch particular lines from a file.

USAGE:

1) cat linenumbers | ./getlines file

2) echo 1 3 100 | ./getlines -n file

NOTE:
1) Line numbers can be separated by space, tab or newline.
2) Line numbers do not need to be sorted. They are sorted internally.
   However, if the line numbers are sorted then you can supply the -n
   flag to prevent sorting.