If main.sh is being sourced instead of being executed it respects the
variables __usage and __helptext being defined beforehand. Its behaviour
will depend on the sourcing script. If __usage is defined but empty no
argument parsing is done. If __helptext is defined but empty no helptext
will be shown.
Logging support still exists as it did before. The environment variables
LOG_LEVEL and NO_COLOR are supported just as they were before.
* better logging: support for multiple lines
* renamed logging function from _fmt to __b3bp_log
* adding example to show how multiline logging works
the main idea is to be able to log e.g. the output of a command and
b3bp taking care of splitting and prefixing every line.
* simplified code, removed hacky space-at-EOL
