added linethrough checkbox and make it optional

browser/main/lib/ConfigManager.js

@@ -65,7 +65,8 @@ export const DEFAULT_CONFIG = {
     smartArrows: false,
     allowCustomCSS: false,
     customCSS: '',
-    sanitize: 'STRICT' // 'STRICT', 'ALLOW_STYLES', 'NONE'
+    sanitize: 'STRICT', // 'STRICT', 'ALLOW_STYLES', 'NONE'
+    lineThroughCheckbox: true
   },
   blog: {
     type: 'wordpress', // Available value: wordpress, add more types in the future plz

browser/main/modals/PreferencesModal/UiTab.js

@@ -107,6 +107,7 @@ class UiTab extends React.Component {
         smartArrows: this.refs.previewSmartArrows.checked,
         sanitize: this.refs.previewSanitize.value,
         allowCustomCSS: this.refs.previewAllowCustomCSS.checked,
+        lineThroughCheckbox: this.refs.lineThroughCheckbox.checked,
         customCSS: this.customCSSCM.getCodeMirror().getValue()
       }
     }
@@ -512,6 +513,16 @@ class UiTab extends React.Component {
               </select>
             </div>
           </div>
+          <div styleName='group-checkBoxSection'>
+            <label>
+              <input onChange={(e) => this.handleUIChange(e)}
+                checked={this.state.config.preview.lineThroughCheckbox}
+                ref='lineThroughCheckbox'
+                type='checkbox'
+              />&nbsp;
+              {i18n.__('Allow line through checkbox')}
+            </label>
+          </div>
           <div styleName='group-checkBoxSection'>
             <label>
               <input onChange={(e) => this.handleUIChange(e)}