_context.py: Add pull_build_trees global user context, the default

of which is set to False via the addition of pullbuildtree to
userconfig.yaml

_frontend/app.py & cli.py: Add --pull-build-trees 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