cli.py: Use Click's show_default for defaults in help text

Tom Mewettrequested to merge
tmewett/cli-defaults into master

Description

Defaults for CLI options are currently manually written in the help string. This MR removes them from that and instead uses the Click show_default argument to click.option.

This merge request, when approved, will close: #1110 (closed)

Further changes

I omitted changing bst shell --use-buildtree because the help text is slightly peculiar and I thought rewriting it was beyond the scope of this MR.

Merge request reports