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/Detail/SnippetNoteDetail.js

@@ -599,12 +599,14 @@ class SnippetNoteDetail extends React.Component {
   }
 
   addSnippet () {
-    const { config } = this.props
+    const { config: { editor: { snippetDefaultLanguage } } } = this.props
     const { note } = this.state
 
+    const defaultLanguage = snippetDefaultLanguage === 'Auto Detect' ? null : snippetDefaultLanguage
+
     note.snippets = note.snippets.concat([{
       name: '',
-      mode: config.editor.snippetDefaultLanguage || 'text',
+      mode: defaultLanguage,
       content: '',
       linesHighlighted: []
     }])
@@ -696,8 +698,6 @@ class SnippetNoteDetail extends React.Component {
 
     const viewList = note.snippets.map((snippet, index) => {
       const isActive = this.state.snippetIndex === index
-      let syntax = CodeMirror.findModeByName(convertModeName(snippet.mode))
-      if (syntax == null) syntax = CodeMirror.findModeByName('Plain Text')
       return <div styleName='tabView'
         key={index}
         style={{zIndex: isActive ? 5 : 4}}

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>))
                 }