Figure.colorbar: Add alias for "Q"

pygmt/src/colorbar.py

@@ -18,6 +18,7 @@
     I="shading",
     J="projection",
     L="equalsize",
+    Q="log",
     R="region",
     V="verbose",
     W="scale",
@@ -110,6 +111,10 @@ def colorbar(self, **kwargs):
         **i** is prepended the interval range is annotated instead. If
         ``shading`` is used each rectangle will have its constant color
         modified by the specified intensity.
+    log : bool
+        Select logarithmic scale and power of ten annotations. All z-values
+        in the CPT will be converted to p = log10(z) and only integer p-values
+        will be annotated using the 10^p format [Default is linear scale].
     zfile : str
         File with colorbar-width per color entry. By default, the width of the
         entry is scaled to the color range, i.e., z = 0-100 gives twice the