Figure.image: Add alias "bitcolor" for "G" (#2615)

pygmt/src/image.py

@@ -7,11 +7,12 @@
 
 @fmt_docstring
 @use_alias(
-    R="region",
-    J="projection",
     D="position",
     F="box",
+    G="bitcolor",
+    J="projection",
     M="monochrome",
+    R="region",
     V="verbose",
     c="panel",
     p="perspective",
@@ -49,6 +50,14 @@ def image(self, imagefile, **kwargs):
         [**+p**\ [*pen*]][**+r**\ [*radius*]][**+s**\ [[*dx*/*dy*/][*shade*]]].
         If set to ``True``, draw a rectangular border around the image
         using :gmt-term:`MAP_FRAME_PEN`.
+    bitcolor : str or list
+        [*color*][**+b**\|\ **f**\|\ **t**].
+        Change certain pixel values to another color or make them transparent.
+        For 1-bit images you can specify an alternate *color* for the
+        background (**+b**) or the foreground (**+f**) pixels, or give no color
+        to make those pixels transparent. Can be repeated with different
+        settings. Alternatively, for color images you can select a single
+        *color* that should be made transparent instead (**+t**).
     monochrome : bool
         Convert color image to monochrome grayshades using the (television)
         YIQ-transformation.