Support plugin invocation with API

src/ocrmypdf/_plugin_manager.py

@@ -16,17 +16,17 @@
 # along with OCRmyPDF.  If not, see <http://www.gnu.org/licenses/>.
 
 import importlib
+from typing import List
 
 import pluggy
 
 from ocrmypdf import pluginspec
 
 
-def get_plugin_manager(options):
+def get_plugin_manager(plugins: List[str]):
     pm = pluggy.PluginManager('ocrmypdf')
     pm.add_hookspecs(pluginspec)
-
-    for name in options.plugins:
+    for name in plugins:
         module = importlib.import_module(name)
         pm.register(module)
     return pm