[#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
ISSUE #2: implement option --output-mode

default value "c" keeps the existing behavior (copy to clipboard)
value "o" 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