[#12]remove extra line

Remove return at beginning of generated comment
[#30] Use gtac instead of tac on osx

tac is not available out of the box so mac users must install
coreutils to get it.
Check that tac|gtac is available
[#2] implement option --standard-output

--standard-output option enables silent mode automatically, requires either option --number or option --grep and print generated comment to the standard output
also:
 - minor fix in help of --number option
 - add default values for all variables initialized from command arguments
 - minor indent fixes in ui_functions.sh
[#2] remove description override in print-mode 0

this functionnality makes print-mode 0 non pipable, it was not used, so it was decided to remove it