Replace --collectors.enabled with per-collector flags (#640)

* Move NodeCollector into package collector

* Refactor collector enabling

* Update README with new collector enabled flags

* Fix out-of-date inline flag reference syntax

* Use new flags in end-to-end tests

* Add flag to disable all default collectors

* Track if a flag has been set explicitly

* Add --collectors.disable-defaults to README

* Revert disable-defaults flag

* Shorten flags

* Fixup timex collector registration

* Fix end-to-end tests

* Change procfs and sysfs path flags

* Fix review comments

collector/uname_linux.go

@@ -22,7 +22,7 @@ import (
 )
 
 var unameDesc = prometheus.NewDesc(
-	prometheus.BuildFQName(Namespace, "uname", "info"),
+	prometheus.BuildFQName(namespace, "uname", "info"),
 	"Labeled system information as provided by the uname system call.",
 	[]string{
 		"sysname",
@@ -38,7 +38,7 @@ var unameDesc = prometheus.NewDesc(
 type unameCollector struct{}
 
 func init() {
-	Factories["uname"] = newUnameCollector
+	registerCollector("uname", defaultEnabled, newUnameCollector)
 }
 
 // NewUnameCollector returns new unameCollector.