Add support for short YAML file extension (#8)

When I tried out Slippers it didn't work initially because
I had created a "components.yml" file instead of "components.yaml".
I didn't see the system check message either because the
log statements already moved it too far up the terminal.

I'm not sure what suffix is more widely used but, for example,
Docker Compose refers its config as "docker-compose.yml" while
suporting both file endings:

> The Compose file is a YAML file defining services, networks and
> volumes. The default path for a Compose file is ./docker-compose.yml.
>
> Tip: You can use either a .yml or .yaml extension for this file.
> They both work.

slippers/apps.py

@@ -3,18 +3,22 @@
 from django.apps import AppConfig
 from django.core.checks import Warning, register
 from django.template.exceptions import TemplateDoesNotExist
-from django.template.loader import get_template
+from django.template.loader import select_template
 from django.utils.autoreload import autoreload_started, file_changed
 
 import yaml
 
 from slippers.templatetags.slippers import register_components
 
 
+def get_components_yaml():
+    return select_template(["components.yaml", "components.yml"])
+
+
 def register_tags():
     """Register tags from components.yaml"""
     try:
-        template = get_template("components.yaml")
+        template = get_components_yaml()
         components = yaml.safe_load(template.template.source)
         register_components(components.get("components", {}))
     except TemplateDoesNotExist:
@@ -24,7 +28,7 @@ def register_tags():
 def watch(sender, **kwargs):
     """Watch when component.yaml changes"""
     try:
-        template = get_template("components.yaml")
+        template = get_components_yaml()
         sender.extra_files.add(Path(template.origin.name))
     except TemplateDoesNotExist:
         pass
@@ -40,7 +44,7 @@ def changed(sender, file_path: PosixPath, **kwargs):
 def checks(app_configs, **kwargs):
     """Warn if unable to find components.yaml"""
     try:
-        get_template("components.yaml")
+        get_components_yaml()
     except TemplateDoesNotExist:
         return [
             Warning(