Discussion about aliases for multi-axis log/power projections

When trying to copy the second GMT tutorial into GMT.jl I have encountered a missing symbol (I think). The GMT command I want to reproduce is:

```
gmt begin GMT_tut_2
gmt basemap -R1/10000/1e20/1e25 -JX9il/6il -Bxa2+l"Wavelength (m)" -Bya1pf3+l"Power (W)" -BWS
gmt end show
```

Both the x and y axis are in a log scale. At the moment, this can be achieved by specifying the `l` flag to the `figsize` strings:

```julia
function tutorial2()
basemap(
region = (1, 10000, 1e20, 1e25),
figsize = ("9il", "6il"),
frame = (
axes = (:left_full, :bottom_full),
xlabel = "Wavelength (m)",
ylabel = "Power (W)",
),
xaxis = (annot = 2, ticks = :auto),
yaxis = (annot = "1p", ticks = 3),
show = true,
Vd = 1,
)
end
```

However, it would be nice to separate projection and size. Maybe there could be ~~distinct `xproj` and `yproj` arguments (larger change, would require rework of existing projection bindings)~~ or just a `:logxy` projection. Let me know which one of these you prefer, and I will look into an implementation (unless you already have something planned).

EDIT: To stay consistent with the map projection syntax, maybe it would be better to use something like `proj = (name = :log, apply = "xy")`.