Adding support to output to reader

[Neutroni]

Currently it isn't possible to capture the output of non file format outputs directly when using the library, to allow using for example SVG outputs without having to write and read temporary files it would be preferable be able to access the output from the calling Rust code.
I have somewhat looked into implementing this and can see two ways this could be implemented. Either by adding support for specifying the stdout for the gnuplot Command::new in show_and_keep_running and having a way to access the process piped stdout from CloseSentinel
Or by adding a new method similar to how show works but instead of returning CloseSentinel the method would return the command output directly.

I have proof of concept available at Neutroni@7bde03a still mostly untested but if either or both implementations seem sensible I can actually test the code and create PR.

[SiegeLord]

I don't fully understand the request. Isn't the SVG terminal a "file format output"? Is the desire here to get terminals that output to file, to output to a Rust reader of some kind (probably not String, but some bytes thing). Why wouldn't it be a change to Figure::set_terminal?

[Neutroni]

I want the output of the gnuplot command so that a web server can send the output directly, currently I'm using fg.set_terminal("svg size 1280,720 dynamic background rgb 'white'", ""); with empty filename so that the output gets send to stdout but that only works with CGI and not a proper web server since the call to Command::new in show_and_keep_running does not specify stdout so the output defaults to Stdio::inherit()
Gnuplot has the ability to send non binary terminals output to stdout when the file is not specified and I want to capture the output directly.
From Gnuplot documentation If the filename is omitted, any output file opened by a previous invocation of set output will be closed and new output will be sent to STDOUT