Change default highlight_group

As I changed colorschemes, I started to notice that CursorLine was the
default color I was originally going for. ColorColumn happened to be the
right one on the colorschemes I had been using, but it wasn't right
across a broader spectrum of colorschemes.

This change may introduce a change to users; I hope it isn't too
jarring. My rationale is twofold:

* the readme suggests using the plugin with hard coded defaults, so many
  users may be doing this, which means this change won't affect them
* CursorLine is the same color as ColorColumn for many colorschemes

Hopefully this ends up being an improvement.

Author: aaronik

README.md

@@ -57,7 +57,7 @@ The swap commands intelligently swap nodes, including comments and attributes/de
 
     -- The color of the above highlight. Must be a valid vim highlight group.
     -- (see :h highlight-group for options)
-    highlight_group = 'ColorColumn',
+    highlight_group = 'CursorLine',
   }
 }
 ```
@@ -79,7 +79,7 @@ use {
 
         -- The color of the above highlight. Must be a valid vim highlight group.
         -- (see :h highlight-group for options)
-        highlight_group = 'ColorColumn',
+        highlight_group = 'CursorLine',
       })
   end
 }
@@ -90,7 +90,7 @@ use {
 Plug 'aaronik/treewalker.nvim'
 
 " This line is optional
-:lua require('treewalker').setup({ highlight = true, highlight_duration = 250, highlight_group = 'ColorColumn' })
+:lua require('treewalker').setup({ highlight = true, highlight_duration = 250, highlight_group = 'CursorLine' })
 ```
 
 ---

lua/treewalker/init.lua

@@ -9,7 +9,7 @@ local Treewalker = {}
 Treewalker.opts = {
   highlight = true,
   highlight_duration = 250,
-  highlight_group = "ColorColumn",
+  highlight_group = "CursorLine",
 }
 
 -- This does not need to be called for Treewalker to work. The defaults are preinitialized and aim to be sane.

tests/treewalker/highlight_spec.lua

@@ -75,7 +75,7 @@ describe("Highlights in a regular lua file: ", function()
     tw.setup({ highlight = true, highlight_duration = 250 })
     tw.move_down()
     local hl_group_arg = highlight_stub.calls[1].refs[3]
-    assert.equal("ColorColumn", hl_group_arg)
+    assert.equal("CursorLine", hl_group_arg)
   end)
 
   it("respects highlight_group config option", function()