Add cli main and user config option for 'pull-buildtrees' context.

_context.py: Add pull_buildtrees global user context, the default
of which is set to False via the addition of pull-buildtrees to
userconfig.yaml cache group.

_frontend/app.py & cli.py: Add --pull-buildtrees as a bst main
option, which when passed will override the default or user defined
context for pull_buildtrees.

tests/completions/completions.py: Update for the added flag.