Updates happen via git with no regular release schedule or versioning.
However, I will bump up the GNUPLOT_IOSTREAM_VERSION macro whenever
something major changes (but note that prior to version 2 this macro is not
defined).

Version 3:
	* New subplots interface allows easily creating many plots in one window.
	  example-data-1d.cc and example-data-2d.cc use this new interface.
	* We now require C++17.  Code has been modernized to use standard metaprogramming
	  functions rather than boost::mpl.
	* Support for Eigen matrix library.

Version 2:
	* Major rewrite.
	* Nearly every combination of container and tuple types is supported.
	* Now you should use send1d() and send2d() rather than just send().
	* When the Gnuplot object is constructed with no arguments, an attempt is made to determine
	  the best gnuplot command and arguments for your operating system.

Version 1:
	* Original version.
	* The GNUPLOT_IOSTREAM_VERSION macro is not defined prior to version 2.
