Added option for language selector

Implemented a language selector submenu that can be turned on or off by a config value.

Resolves: panr/hugo-theme-terminal#31

README.md

@@ -78,6 +78,8 @@ paginate = 5
   themeColor = "orange"
   # if you set this to 0, only submenu trigger will be visible
   showMenuItems = 2
+  # show selector to switch language
+  showLanguageSelector = false
   # set theme to full screen width
   fullWidthTheme = false
   # center theme with default width