Add configuration to select layouts to cycle through

[sathishmanohar]

Currently selecting the default layout is configurable via

merge_tool = {
      layout = "diff3_horizontal",
    },

When we cycle through layout we cycle through all available layouts. It would be great if I can select which layouts I want to cycle through by a config option. I don't know. Something like would be great.

merge_tool = {
      layout = "diff1_plain",
      selected_layouts = { "diff1_plain", "diff3_vertical", "diff3_mixed", "diff4_mixed" }
    },