Add cli main & user conf option for 'cache-buildtrees' context

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

app.py & cli.py: Add --cache-buildtrees/--no-cache-buildtrees as
a bst main option, which when passed can override the default or
user defined context for cache_buildtrees.

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