Add colorcolumn option

Fixes #333

For example: `> set colorcolumn 80`.

runtime/help/colors.md

@@ -108,6 +108,8 @@ Here is a list of the colorscheme groups that you can use:
 * gutter-error
 * gutter-warning
 * cursor-line
+* current-line-number
+* color-column
 
 Colorschemes can be placed in the `~/.config/micro/colorschemes` directory to be used.
 

runtime/help/options.md

@@ -23,6 +23,11 @@ Here are the options that you can set:
     You can read more about micro's colorschemes in the `colors` help topic
     (`help colors`).
 
+* `colorcolumn`: if this is not set to 0, it will display a column at the specified
+   column. This is useful if you want column 80 to be highlighted special for example.
+
+	default value: `0`
+
 * `tabsize`: sets the tab size to `option`
 
 	default value: `4`