Add Sources field to Config (#501)

* Add Sources field to Config

and keep track of merged files there. Also print the Sources as a
comment in the String() method.

Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>

* Fix tests

Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>

* Fix linter

Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>

* Fix TODO

by renaming the toMap function and making it operate on ConfigValues
instead of full Config objects (because after all, it wasn't copying the
Sources field)

Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>

* [minor] Return ConfigValues interface when erroring out

although nobody should consume it since we errored

Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>

* Add check for "Sources" comment

to check that these all generate a line:
- cmdline
- remote config (config_url)
- local files

Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>

---------

Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>

collector/suite_test.go

@@ -11,6 +11,7 @@ import (
 )
 
 func TestConfig(t *testing.T) {
+	// format.TruncatedDiff = false
 	RegisterFailHandler(Fail)
 	RunSpecs(t, "Config Collector Suite")
 }