fix: allow config file to load showLineNumbers

- Added in a fix to allow the `showLineNumbers` config option to
  be loaded from the config file
- Previously it seems unclear on how to setup the `showLineNumbers` key
  within the config, and while there was support added for configuring
  the `showLineNumbers` flag, it couldn't be read from the config file,
  since the `validateOptions` method would not retrieve the key from
  the config file
- Hence changed it so that it does retrieve the `showLineNumbers` key
  from the config file, and use it as needed
- Updated the README.md to also provide an example on how to setup
  both the keys

README.md

@@ -184,6 +184,10 @@ pager: true
 width: 80
 # show all files, including hidden and ignored.
 all: true
+# show line numbers (TUI-mode only)
+showLineNumbers: false
+# preserve newlines in the output
+preserveNewLines: false
 ```
 
 ## Feedback

main.go

@@ -148,6 +148,7 @@ func validateOptions(cmd *cobra.Command) error {
 	pager = viper.GetBool("pager")
 	showAllFiles = viper.GetBool("all")
 	preserveNewLines = viper.GetBool("preserveNewLines")
+	showLineNumbers = viper.GetBool("showLineNumbers")
 
 	// validate the glamour style
 	style = viper.GetString("style")