Ability to set custom start / end frames (#14)

* Accept start / end frames in job submissions. Start / end frame support for Blender

* Remove old render_all_frames variables and misc cleanup

* Client work - Client determines frame count for FFMPEG and shows frame picker UI

lib/workers/ffmpeg_worker.py

@@ -17,10 +17,7 @@ class FFMPEGRenderWorker(BaseRenderWorker):
                                                      input_path, "-map", "0:v:0", "-c", "copy", "-f", "null", "-y",
                                                      "/dev/null"], stderr=subprocess.STDOUT).decode('utf-8')
         found_frames = re.findall('frame=\s*(\d+)', stream_info)
-        self.total_frames = found_frames[-1] if found_frames else '-1'
-        self.frame = 0
-
-        # Stats
+        self.project_length = found_frames[-1] if found_frames else '-1'
         self.current_frame = -1
 
     def generate_worker_subprocess(self):