npm: updated to 6.8.0

v6.8.0:

This release includes an implementation of [RFC 10], documenting an optional field that can be used to specify
the directory path for a package within a monorepo.

NEW FEATURES
* Update package.json docs to include repository.directory details.

BUGFIXES
* Add @types to ignore list to fix git clean -fd.
* Fix common.npm callback arguments.
* Show installed but unmet peer deps.
* Use figgy-config to make sure extra opts are there.
* Fix ls-collaborators access error for non-scoped case.
* Fix issue with sub-folder local references.

DEPENDENCY BUMPS
* npm-registry-couchapp@2.7.1
* npm-registry-fetch@3.9.0:
* Make sure publishing with legacy username:password _auth works again.
* pacote@9.4.1
* normalize-package-data@2.5.0
* npm-packlist@1.3.0
* read-package-tree@5.2.2

MISC
* Use const in lib/fetch-package-metadata.md.
* Replace ronn with marked-man in .npmignore.
* Reduce work to test if executable ends with a 'g'.

(adam)