fix: add :enabled sub-config for fixer tools
This allows distributing opt-in fixer tool configurations in .hgrc
files. This may be useful for adding default configs in core, or for
orgranizations that want to provide configs to their users.
Tools are still enabled by default because it would be confusing to add a
config and find that it has no effect until you add enabled=true.
Differential Revision: https://phab.mercurial-scm.org/D6975