Support `-` as filename to read from/to stdin

[septatrix]

I often use this tool to pipe SVG into it (and sometimes pipe the resulting PDF into another command but that is more rare). It would be great if this tool followed the convention to interpret - as reading from stdin (and outputting to there). Reading from/writing to files of such name would still be possible using ./- (i.e. open file named - in current dir ./)