Treat "-" as stdin

[sp1ritCS]

If Clapper is invoked with "-" as file, it should read from stdin (like other GNU applications), rather than trying to read "$PWD/-".

This allows for more straight forward piping:

streamlink twitch.tv/pimpcsgo 720p60 --stdout | com.github.rafostar.Clapper -

rather than

[...] | com.github.rafostar.Clapper /dev/stdin

[Rafostar]

This is gonna depend on finishing and merging gstplaylist branch, as setting and parsing playlist files will be handled a little differently then.