Default program open assumes MacOS

[edwardcwang]

https://github.com/chick/visualizer/blob/master/src/main/scala/dotvisualizer/FirrtlDiagrammer.scala#L175

Running open on Linux results in a confusing error message:

Couldn't get a file descriptor referring to the console
[error] (run-main-0) java.lang.RuntimeException: Nonzero exit value: 1
java.lang.RuntimeException: Nonzero exit value: 1
    at scala.sys.package$.error(package.scala:27)
    at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.slurp(ProcessBuilderImpl.scala:134)
    at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.$bang$bang(ProcessBuilderImpl.scala:104)
    at dotvisualizer.FirrtlDiagrammer$.show(FirrtlDiagrammer.scala:67)
    at dotvisualizer.FirrtlDiagrammer$.run(FirrtlDiagrammer.scala:177)

This is because open on Linux does something very different:

$ open --help
Usage: open [OPTIONS] -- command

This utility help you to start a program on a new virtual terminal (VT).