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

docs/genqlient.yaml

@@ -8,15 +8,17 @@
 #  schema:
 #  - user.graphql
 #  - ./schema/*.graphql
-#  - ./another_directory/**/*.graphqls
+#  - ./another_directory/*/*.graphql
 schema: schema.graphql
 
-# Filenames or globs with the operations for which to generate code, relative
+# Filename(s) or globs with the operations for which to generate code, relative
 # to genqlient.yaml.
 #
 # These may be .graphql files, containing the queries in SDL format, or
 # Go files, in which case any string-literal starting with (optional
 # whitespace and) the string "# @genqlient" will be extracted as a query.
+#
+# Like schema, this may be a single file or a list.
 operations:
 - genqlient.graphql
 - "pkg/*.go"