allow setting stdin, stdout, stderr on calls to RunCommand*

The RunCommandWithClearEnvironment and RunCommand methods now support
setting the stdin_fd, stdout_fd, and stderr_fd attach_options. This
allows programs to specify pipes for communication with the attached
process.
Merge pull request #6 from kelseyhightower/set_file_descriptors_run_command

allow setting stdin, stdout, stderr on calls to RunCommand*