Take input files from stdin (standard input)

[desertwitch]

This extends upon #273 and #279.

I always missed this in par2cmdline and since the logic is already in place,
it would now be nice to use the error-case where @ is followed by nothing
for taking the input from standard input, arguably a common Linux use-case.

This would allow for setups like:

desertwitch@vDebianDev:~/par2cmdline$ cat test.txt | ./par2 c test.par2 @
Block size: 16
Source file count: 2
Source block count: 1506
Recovery block count: 75
Recovery file count: 7

Opening: README.md
Opening: ROADMAP
Computing Reed Solomon matrix.
Constructing: done.
Wrote 1200 bytes to disk
Writing recovery packets
Writing verification packets
Done

And especially use in shell-scripting feeding other command's output into par2cmdline.

[Rhialto]

For the case of reading from standard input, it is more common on Unix to use - as the file name. So you'd have @-.
@/dev/stdin would also work, if you have that special file on your system.

[desertwitch]

I used @ because it eliminates an existing error path and requires no additional code to handle.
Arguably the cleanest would be using - instead of @ or @-...
I'd be happy either way, as long as the possibility to use standard input is implemented.

[BlackIkeEagle]

Interesting discussion, but currently I'm leaning more to how it is implemented now, since - usually implies you are streaming some data in, if we use - for the filelist only it might be confusing