initial implementation of --explain

Currently it only displays explanations of options like --in and --copies.

In the future, it should explain preferred content expression evaluation
and other decisions.

The explanations of a few things could be better. In particular,
"standard" will just appear as-is (or as "!standard" if it doesn't
match), rather than explaining why the standard preferred content expression
for the group matches or not.

Currently as implemented, it goes to stdout, and so commands like
git-annex find that have custom output will not display --explain
information. Perhaps that should change, dunno.

Sponsored-by: Dartmouth College's DANDI project

Annex.hs

@@ -122,6 +122,7 @@ data AnnexRead = AnnexRead
 	, transferrerpool :: TransferrerPool
 	, debugenabled :: Bool
 	, debugselector :: DebugSelector
+	, explainenabled :: Bool
 	, ciphers :: TMVar (M.Map StorableCipher Cipher)
 	, fast :: Bool
 	, force :: Bool
@@ -152,6 +153,7 @@ newAnnexRead c = do
 		, transferrerpool = tp
 		, debugenabled = annexDebug c
 		, debugselector = debugSelectorFromGitConfig c
+		, explainenabled = False
 		, ciphers = cm
 		, fast = False
 		, force = False