Make MathJax support optional

to enable add following to book.toml
```toml
[output.html]
mathjax-support = true
```

src/book/mod.rs

@@ -498,6 +498,23 @@ impl MDBook {
         false
     }
 
+    pub fn with_mathjax_support(mut self, mathjax_support: bool) -> Self {
+        if let Some(htmlconfig) = self.config.get_mut_html_config() {
+            htmlconfig.set_mathjax_support(mathjax_support);
+        } else {
+            error!("There is no HTML renderer set...");
+        }
+        self
+    }
+
+    pub fn get_mathjax_support(&self) -> bool {
+        if let Some(htmlconfig) = self.config.get_html_config() {
+            return htmlconfig.get_mathjax_support();
+        }
+
+        false
+    }
+
     pub fn get_google_analytics_id(&self) -> Option<String> {
         if let Some(htmlconfig) = self.config.get_html_config() {
             return htmlconfig.get_google_analytics_id();

src/config/htmlconfig.rs

@@ -7,6 +7,7 @@ pub struct HtmlConfig {
     destination: PathBuf,
     theme: PathBuf,
     curly_quotes: bool,
+    mathjax_support: bool,
     google_analytics: Option<String>,
     additional_css: Vec<PathBuf>,
     additional_js: Vec<PathBuf>,
@@ -30,6 +31,7 @@ impl HtmlConfig {
             destination: root.clone().join("book"),
             theme: root.join("theme"),
             curly_quotes: false,
+            mathjax_support: false,
             google_analytics: None,
             additional_css: Vec::new(),
             additional_js: Vec::new(),
@@ -51,6 +53,10 @@ impl HtmlConfig {
             self.curly_quotes = curly_quotes;
         }
 
+        if let Some(mathjax_support) = tomlconfig.mathjax_support {
+            self.mathjax_support = mathjax_support;
+        }
+
         if tomlconfig.google_analytics.is_some() {
             self.google_analytics = tomlconfig.google_analytics;
         }
@@ -116,6 +122,14 @@ impl HtmlConfig {
         self.curly_quotes = curly_quotes;
     }
 
+    pub fn get_mathjax_support(&self) -> bool {
+        self.mathjax_support
+    }
+
+    pub fn set_mathjax_support(&mut self, mathjax_support: bool) {
+        self.mathjax_support = mathjax_support;
+    }
+
     pub fn get_google_analytics_id(&self) -> Option<String> {
         self.google_analytics.clone()
     }

src/config/tomlconfig.rs

@@ -25,6 +25,7 @@ pub struct TomlHtmlConfig {
     pub theme: Option<PathBuf>,
     pub google_analytics: Option<String>,
     pub curly_quotes: Option<bool>,
+    pub mathjax_support: Option<bool>,
     pub additional_css: Option<Vec<PathBuf>>,
     pub additional_js: Option<Vec<PathBuf>>,
 }

src/renderer/html_handlebars/hbs_renderer.rs

@@ -318,6 +318,10 @@ fn make_data(book: &MDBook) -> Result<serde_json::Map<String, serde_json::Value>
         data.insert("google_analytics".to_owned(), json!(ga));
     }
 
+    if book.get_mathjax_support() {
+        data.insert("mathjax_support".to_owned(), json!(true));
+    }
+
     // Add check to see if there is an additional style
     if book.has_additional_css() {
         let mut css = Vec::new();

src/theme/index.hbs

@@ -27,8 +27,10 @@
         <link rel="stylesheet" href="{{this}}">
         {{/each}}
 
+        {{#if mathjax_support}}
         <!-- MathJax -->
         <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
+        {{/if}}
 
         <!-- Fetch Clipboard.js from CDN but have a local fallback -->
         <script src="https://cdn.jsdelivr.net/clipboard.js/1.6.1/clipboard.min.js"></script>