Show plots in internal VS Code plot pane

With this change, when Gnuplot.jl is used within VSCode, the plot
shows in VSCode plot pane instead of in a separate Gnuplot window.

Note that when compared to IJulia and Juno, we do not test for active
VS Code connection. I don't think it is necessary, but if it turns out
to be needed later, it can be done with

    isopen(VSCodeServer.conn_endpoint[])

README.md

@@ -6,7 +6,7 @@
 [![DocumentationStatus](https://img.shields.io/badge/docs-stable-blue.svg?style=flat)](https://gcalderone.github.io/Gnuplot.jl/v1.3.0/)
 
 
-**Gnuplot.jl** is a simple package able to send both data and commands from Julia to an underlying [gnuplot](http://gnuplot.sourceforge.net/) process.  Its main purpose it to provide a fast and powerful data visualization framework, using an extremely concise Julia syntax. It also has automatic display of plots in both Jupyter and Juno.
+**Gnuplot.jl** is a simple package able to send both data and commands from Julia to an underlying [gnuplot](http://gnuplot.sourceforge.net/) process.  Its main purpose it to provide a fast and powerful data visualization framework, using an extremely concise Julia syntax. It also has automatic display of plots in Jupyter, Juno and VS Code.
 
 
 ## Installation

src/Gnuplot.jl

@@ -236,11 +236,12 @@ const sessions = OrderedDict{Symbol, Session}()
 const options = Options()
 
 function __init__()
-    # Check whether we are running in an IJulia, Juno or Pluto session.
+    # Check whether we are running in an IJulia, Juno, VSCode or Pluto session.
     # (copied from Gaston.jl).
     options.gpviewer = !(
         ((isdefined(Main, :IJulia)  &&  Main.IJulia.inited)  ||
          (isdefined(Main, :Juno)    &&  Main.Juno.isactive()) ||
+         (isdefined(Main, :VSCodeServer)) ||
          (isdefined(Main, :PlutoRunner)) )
     )
 end