Igv tools stdout

Support writing output to stdout, instead of output file. This is done by using the string "stdout" for the output file.

Only works for "sort" command, and "count" if the output is wiggle. It could be done for "index" but I have no idea why anyone would want that. 

Writing TDF files to a stream is not as easy because the index position is only known once the whole file is generated. So the output needs to be buffered.