Add custom right-click context menu

[moe93]

Closes #530

Usage:

if (ImPlot::BeginPlot("My Plot", ImVec2(-1, 0), ImPlotFlags_NoCentralMenu))
{
    // Plot things before

    if (ImPlot::BeginCustomContext())
    {
        if (ImGui::MenuItem("My Custom Item")) doThing();
        ImPlot::EndCustomContext(true); // true = append standard menu
    }
    ImPlot::EndPlot();

    // Plot things after
}

This is the work of @PapaNaxos. I just packaged it and created the PR.