Further fixes to gtk version

Improve stability of gtk window.

Make sure release is picked up by code.dlang.org

Donnot strip gtk builds. Stripping seems to cause problems when opening

multiple windows

Testing travis deployment

This release is only to test building release files with travis. Nothing else was changed.

Use ggplotd as the backend and changes to commandline interface

During this update we switched from using cairoD directly to building on ggplotd for the drawing. We also revamped the command line options. This means that you cannot use plotcli with the same arguments as before. This change was made because the new version more directly maps to ggplotd design, making it easier to support all the power of ggplotd, just from the command line.

As a side benefit it also allows us more freedom, because it enables us to use the same column both for x data and for y data, which allows us much more freedom in how to create multiple plots. I.e. if I want to plot column 1 versus column 2 and column 3 versus column 1, this is now possible.

Wider compiler support

Added testing support for more compilers (ldc and gdc) courtesy of @MartinNowak