support for prefixing with a highlight group.

t/section.vim

@@ -8,6 +8,10 @@ describe 'section'
     call airline#parts#define_text('text', 'text')
     call airline#parts#define_raw('raw', 'raw')
     call airline#parts#define_function('func', 'SectionSpec')
+    call airline#parts#define('hi', {
+          \ 'raw': 'hello',
+          \ 'highlight': 'hlgroup',
+          \ })
   end
 
   it 'should create sections with no separators'
@@ -24,5 +28,10 @@ describe 'section'
     let s = airline#section#create_right(['text', 'text'])
     Expect s == '%{airline#util#prepend("text")}%{"text"}'
   end
+
+  it 'should prefix with highlight group if provided'
+    let s = airline#section#create(['hi'])
+    Expect s == '%#hlgroup#hello'
+  end
 end