Pyinstaller support (#93)

* Add main.spec

* Fix issue where fetching supported extensions would crash with no default installation

src/engines/core/base_engine.py

@@ -9,12 +9,11 @@ SUBPROCESS_TIMEOUT = 5
 class BaseRenderEngine(object):
 
     install_paths = []
-    supported_extensions = []
 
     def __init__(self, custom_path=None):
         self.custom_renderer_path = custom_path
         if not self.renderer_path() or not os.path.exists(self.renderer_path()):
-            raise FileNotFoundError(f"Cannot find path to renderer for {self.name()} instance")
+            raise FileNotFoundError(f"Cannot find path to renderer for {self.name()} instance: {self.renderer_path()}")
 
         if not os.access(self.renderer_path(), os.X_OK):
             logger.warning(f"Path is not executable. Setting permissions to 755 for {self.renderer_path()}")
@@ -43,6 +42,9 @@ class BaseRenderEngine(object):
     def version(self):
         raise NotImplementedError("version not implemented")
 
+    def supported_extensions(self):
+        return []
+
     @staticmethod
     def downloader():  # override when subclassing if using a downloader class
         return None
@@ -51,7 +53,8 @@ class BaseRenderEngine(object):
     def worker_class():  # override when subclassing to link worker class
         raise NotImplementedError("Worker class not implemented")
 
-    def ui_options(self):  # override to return options for ui
+    @staticmethod
+    def ui_options(system_info):  # override to return options for ui
         return {}
 
     def get_help(self):  # override if renderer uses different help flag