move highlighter code to correct place and test.

t/highligher.vim

@@ -0,0 +1,10 @@
+describe 'highlighter'
+  it 'should create separator highlight groups'
+    hi Foo1 ctermfg=1 ctermbg=2
+    hi Foo2 ctermfg=3 ctermbg=4
+    call airline#highlighter#add_separator('Foo1', 'Foo2', 0)
+    let hl = airline#highlighter#get_highlight('Foo1_to_Foo2')
+    Expect hl == [ '', '', '4', '2', '' ]
+  end
+end
+