Support multiple set/get/deletes in npm config

[isaacs]

While digging into #2300, I realized it would be a lot easier if we
could do this:

npm config set [email protected] _auth=xxxx

and avoid the whole issue of what gets set first.

Also, why not let npm config get foo bar baz return just the keys
specified?

Also updates the docs, including the statement that npm config set foo
with no value sets it to true, when as far as I can tell, that has
never been the case.

References

[npm-deploy-user]

	angular-quickstart			app-large			app-medium			ember-quickstart			react-app
	prev	current	status	prev	current	status	prev	current	status	prev	current	status	prev	current	status
initial install	41s	40.9s	✅	39.5s	45.6s	🛑	34.5s	33.1s	✅	28.1s	27.6s	✅	33.5s	37.2s	✅🐌
repeat install	9.6s	3s	✅	8.6s	3.4s	✅	8.3s	2.5s	✅	7.7s	1.5s	✅	9.1s	2.6s	✅
with warm cache	32.3s	23.4s	✅	33.8s	27.1s	✅	31.3s	19.7s	✅	23.4s	15.9s	✅	29.1s	18.6s	✅
with node_modules	9.2s	5.3s	✅	8.6s	4.4s	✅	9s	3.3s	✅	7.8s	1.8s	✅	9.4s	5.5s	✅
with lockfile	32.6s	28.9s	✅	31s	29.9s	✅	29s	23.5s	✅	21.7s	17.9s	✅	27.4s	24.8s	✅
with warm cache and node_modules	9.3s	3.5s	✅	7.9s	4.3s	✅	8.4s	10.8s	🛑	7.6s	1.8s	✅	9.2s	13.3s	🛑
with warm cache and lockfile	24.7s	9.4s	✅	26.3s	11.7s	✅	24.3s	7.3s	✅	17.8s	5.8s	✅	21.3s	7.3s	✅
with node_modules and lockfile	10.1s	4.9s	✅	9.8s	3.9s	✅	8.6s	2.9s	✅	7.8s	1.6s	✅	9.7s	4.8s	✅

[nlf]

looks good, nice little usability win here 🎉