postprocess: support for finding ffmpeg in same directory

this will be helpful on windows if you dont have ffmpeg in
your PATH. it will look for ffmpeg in same directory
as svtplay-dl.exe.

lib/svtplay_dl/postprocess/__init__.py

@@ -1,7 +1,9 @@
 import logging
 import os
+import pathlib
 import platform
 import re
+import sys
 from json import dumps
 from random import sample
 from shutil import which
@@ -24,6 +26,10 @@ class postprocess:
             self.detect = which(i)
             if self.detect:
                 break
+        if self.detect is None and platform.system() == "Windows":
+            path = pathlib.Path(sys.executable).parent / "ffmpeg.exe"
+            if path.is_file:
+                self.detect = path
 
     def merge(self):
         if self.detect is None: