new feature: auto detect snippet language

only try to detect after pasting and mode has not been set and default snippet language is "Auto Detect"

browser/main/modals/PreferencesModal/UiTab.js

@@ -481,6 +481,7 @@ class UiTab extends React.Component {
                 ref='editorSnippetDefaultLanguage'
                 onChange={(e) => this.handleUIChange(e)}
               >
+                <option key='Auto Detect' value='Auto Detect'>Auto Detect</option>
                 {
                   _.sortBy(CodeMirror.modeInfo.map(mode => mode.name)).map(name => (<option key={name} value={name}>{name}</option>))
                 }