Added option to site Params for customizing the theme selection icon.

[petebannister]

I was building a help system with light/dark theme selection and preferred a different icon for the theme selector (fa-adjust). This PR makes it possible to set the font-awesome class in the site configuration params using e.g.

[params]
  themeSelectIcon = "fa-adjust"

[McShelby]

Thanks for submitting the patch. It's a valuable addtion to the theme's features.

There are a few things to change before adding this to the theme:

• For consistency it is preferable to work as in the icon / badge shortcode:

  If an icon name is configured without the fa- prefix the theme automatically prepends it with fa-fw fas fa-

  If an fa- prefix is found in the configured icon name, the theme doesn't add anything and puts everything into your hand.

  This gives you the maximum versatility without dropping convenience for simple cases.

• For completeness, this mechanism should also be applied to the language picker, home and clear-history icon

If you provide the changes for the first bullet point to the theme selection icon, I would implement it for the addtional icons of the second bullet point.