fix: allow config file to load showLineNumbers

[prithvijj]

Changes

• 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

Testing Notes

Ran

go build
./glow config

# added in `preserveNewLines: true` and `showLineNumbers: true` key 

and for testing purposes to see if the config option was being read added in

func validateOptions(cmd *cobra.Command) error {
	// grab config values from Viper
	width = viper.GetUint("width")
	mouse = viper.GetBool("mouse")
	pager = viper.GetBool("pager")
	showAllFiles = viper.GetBool("all")
	preserveNewLines = viper.GetBool("preserveNewLines")

+	panic(fmt.Sprintf("preserveNewLines: %v, showLineNumbers: %v", preserveNewLines, showLineNumbers))

to showcase the configurations are being read

Previously:
When preserveNewLines: true and showLineNumbers: true within the config, the panic output shows that
preserveNewLines is being recognized, but showLineNumbers is not being recognized, which is unexpected

When ./glow -l is used, the panic output shows that the preserveNewLines is being recognized, and showLineNumbers is being recognized which is expected

---

After:

When preserveNewLines: true and showLineNumbers: true within the config, the panic output shows that
preserveNewLines is being recognized and showLineNumbers is being recognized, which is expected

When preserveNewLines: true and showLineNumbers: false within the config, the panic output shows that
preserveNewLines is being recognized and showLineNumbers is being recognized, which is expected

Removed the panic line used for testing purposes, ran go build; ./glow then navigated to the README.md to show that the line numbers are being shown, since the config has showLineNumbers: true

Other Notes

Related to #652

• Any tests that I can write?