Add sample for color switch in colors docs (#274)

One of the things I used for a current project was a switch to change the color schemes quickly. I added the snippet in [Specifying Colors & Gradients](https://squib.readthedocs.io/en/latest/colors.html) and in the wiki: https://github.com/andymeneely/squib/wiki/Switch-card-background-or-invert-theme.


* Add sample for color switch in colors docs

* Update docs/colors.rst

Co-Authored-By: Andy Meneely <andy.meneely@gmail.com>

* Update docs/colors.rst

Co-Authored-By: Andy Meneely <andy.meneely@gmail.com>

* Update docs/colors.rst

Co-Authored-By: Andy Meneely <andy.meneely@gmail.com>

* Update docs/colors.rst

Co-Authored-By: Andy Meneely <andy.meneely@gmail.com>

* Update docs/colors.rst

Co-Authored-By: Andy Meneely <andy.meneely@gmail.com>

* Update docs/colors.rst

Co-Authored-By: Andy Meneely <andy.meneely@gmail.com>

* Add color switch sample and update docs

* Add color switch sample and update docs

samples/colors/_switch_color.rb

@@ -0,0 +1,33 @@
+require 'squib'
+
+# Choose between black and white color theme for type snake
+#   * Allow using white snake cards with black text or
+#     black snake cards with white text
+color = 'white'
+
+cards = Squib.csv file: '_switch_color_data.csv', col_sep: "\t"
+
+Squib::Deck.new cards: cards['Type'].size do
+
+    background_color = cards['Type'].map do |t|
+        if color == 'black' && t == "Snake" then
+            "black"
+        else
+            "white"
+        end
+    end
+    background color: background_color
+
+    text_color = cards['Type'].map do |t|
+        if color == 'black' && t == "Snake" then
+            "white"
+        else
+            "black"
+        end
+    end
+
+    text str: cards['Text'], color: text_color
+
+    save_png prefix: '_switch_color_sample_'
+
+end

samples/colors/_switch_color_data.csv

@@ -0,0 +1,3 @@
+Type	Text
+Snake	This is a snake.
+Lion	This is a lion.