Refactor query reading from file, update docs

Add support for @file syntax in more places.

cmd/snapshot_pull.go

@@ -88,12 +88,11 @@ func aptlySnapshotPull(cmd *commander.Command, args []string) error {
 	// Initial queries out of arguments
 	queries := make([]deb.PackageQuery, len(args)-3)
 	for i, arg := range args[3:] {
-		var q string
-		q, err = getContent(arg)
+		value, err := GetStringOrFileContent(arg)
 		if err != nil {
 			return fmt.Errorf("unable to read package query from file %s: %w", arg, err)
 		}
-		queries[i], err = query.Parse(q)
+		queries[i], err = query.Parse(value)
 		if err != nil {
 			return fmt.Errorf("unable to parse query: %s", err)
 		}
@@ -172,6 +171,8 @@ versions from <source> following dependencies. New snapshot <destination>
 is created as a result of this process. Packages could be specified simply
 as 'package-name' or as package queries.
 
+Use '@file' syntax to read package queries from file and '@-' to read from stdin.
+
 Example:
 
     $ aptly snapshot pull wheezy-main wheezy-backports wheezy-new-xorg xorg-server-server