[#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
Add link to Gitter

Add a link to the gitter chat room to provide a better support
[#2] fix options failing if after --standard-output option

--standard-output had a shift command causing next commande line option to be swallowed
 ex.: commande 'jirac --standard-output -n 1' failed with "missing option --number" message
[#24] unify logging to console

refacto every message to the console to use jirac_log for consistency
add a log-level option to set a level of details for JIRAC logs to the console : 0 = ERROR, 1 = INFO, 2 = DEBUG
option silent-mode is now implemented as setting the log level to -1 (ie. no log at all)
remove JIRAC header and victory ASCII arts because they are no logs and can not be controlled with log-level
refacto all program exits paths to use the same functions
move exit functions to core_functions.sh for consistency (ie. with the other log and exit related functions)
inline conditions, fix indent in few places, remove useless [[]]
[[]] actually required for == operator with pattern matching
[#1] display selected commit hash and subject in INFO in print-mode 1