Always use iplayer version as default if found
Added larger thumbnail sizes

New values for --thumbsize:

By width:

1024 => '1024x576',
1280 => '1280x720',
1600 => '1600x900',
1920 => '1920x1080',

By number:

8  => '1024x576',
9  => '1280x720',
10 => '1600x900',
11 => '1920x1080',
Revamped replacement of missing default version by iplayer version