Align support for multiple operation-files and multiple schema-files (#137)

Multiple schema-files are now supported as of #134, but the support was
a bit different from how we did multiple operation-files.  Before anyone
starts to depend on the ways the syntaxes differ, let's just make them
the same.  Since it's easy, I also added support for having just a
single operations-file.

I also realized while writing this that the type-change is technically
breaking (if you call from Go), so documented it as such. I think
this is unlikely to affect many people.

Test plan: make check

generate/stringlist.go

@@ -22,12 +22,3 @@ func (a *StringList) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	*a = multi
 	return nil
 }
-
-func (a StringList) Has(file string) bool {
-	for _, existing := range a {
-		if existing == file {
-			return true
-		}
-	}
-	return false
-}