Porcelain driver that offers additional features over the basic one.

Users are not supposed to call functions in this module directly. Use functions in Porcelain instead.

This driver will be used by default if it can locate the external program named goon in the executable path. If goon is not found, Porcelain will fall back to the basic driver.

The additional functionality provided by this driver is as follows:

• ability to signal EOF to the external program
• send an OS signal to the program
• (to be implemented) more efficient piping of multiple programs