maint: make gen-contributors-list.sh suitable for generation of CREDITS

maint: make gen-contributors-list.sh suitable for generation of CREDITS

Implement various features in order to make it suitable for the
generation of the CREDITS file:
- add an option for including contributor e-mail;
- add an option to read additional list of contributors from stdin;
- add shorthand for referring to the initial commit of the branch;
- document all of the above in a help message.

* maint/gen-contributors-list.sh (print_help): New function.
(script): Rename to SCRIPT.
(SCRIPT_NORM_EMAILS, MATCH_OUT, OUT_EMAILS, OUT_NO_EMAILS, read_stdin,
include_email): New variables.
Add parsing of options, rewrite input processing.