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 'always' via the addition of
cache-buildtrees to userconfig.yaml cache group. 'failure' & 'never'
can be given as valid options.

app.py & cli.py: Add --cache-buildtrees as a bst main option, which
when passed with a valid option can override the default or user
defined context for cache_buildtrees.

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