Fix bug in customized Windows install
Fix bug in customized Windows install

args/config: set the default values for `ffmpeg-ffmpeg` and `rtmpdump` in the Windows streamlinkrc so if either section is skipped during the install everything will work as expected.

javiercantero/streamlink,back-to/streamlink,chhe/streamlink,bastimeyer/streamlink,streamlink/streamlink,beardypig/streamlink,beardypig/streamlink,back-to/streamlink,mmetak/streamlink,mmetak/streamlink,streamlink/streamlink,gravyboat/streamlink,melmorabity/streamlink,melmorabity/streamlink,wlerin/streamlink,javiercantero/streamlink,wlerin/streamlink,gravyboat/streamlink,bastimeyer/streamlink,chhe/streamlink