⚡️ Accept array of string as valid input schema path. (#134)

* ⚡️ Fixes #88. Accept array of string as valid input schema path.

docs/genqlient.yaml

@@ -4,6 +4,11 @@
 
 # The filename with the GraphQL schema (in SDL format), relative to
 # genqlient.yaml.
+# This can also be a list of filenames, such as:
+#  schema:
+#  - user.graphql
+#  - ./schema/*.graphql
+#  - ./another_directory/**/*.graphqls
 schema: schema.graphql
 
 # Filenames or globs with the operations for which to generate code, relative