Store theme version in a data file

Close #593

data/academic.toml

@@ -0,0 +1,3 @@
+# Academic
+
+version = "2.4.0"

layouts/partials/header.html

@@ -5,7 +5,7 @@
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <meta http-equiv="X-UA-Compatible" content="IE=edge">
-  <meta name="theme" content="hugo-academic">
+  <meta name="generator" content="Source Themes Academic {{ .Site.Data.academic.version }}">
   {{ .Hugo.Generator }}
   {{ with .Site.Params.name }}<meta name="author" content="{{ . }}">{{ end }}