To be more adaptive to user needs, keep_by_hours option works as the
inverse of the "delete_by_hours" option. "keep_by_hours" will keep
any tag newer than the specified number of hours.
Providing passwords via command line is insecure. Adding an option to read
the password from the TTY or stdin. When reading from a TTY, getpass() is
used so that the password is not displayed as it is entered. Getpass()
also displays a prompt, since TTY usage is typically interactive. When
reading from something other than a TTY (e.g. from a pipe or input
redirection), a plain read from stdin is done (without getpass(); without
any promting). This allows the user to pipe the password from some type of
password management/entry tool to registry.py.
* reworked tag selection routine
** "tags-like" parameter adds regexp like filtering
** "keep-tags" parameter specifies tag names that are to keep, regardless of selection
** "keep-tags-like" dito, but regexp based
The keep tags routine works by comparing needed manifest digests - they are read and compared before deleting a manifest.
