Map <C-L> to clear search highlighting

<C-L> normally redraws the screen, so turning off search highlighting is
a natural addition.  But some people like it for <C-W>l, so don't
override it if it's already been mapped.

Note that hlsearch is currently left off by default.

plugin/sensible.vim

@@ -23,6 +23,9 @@ set ttimeoutlen=50
 
 set incsearch
 set smartcase
+if maparg('<C-L>', 'n') ==# ''
+  nnoremap <silent> <C-L> :nohls<CR><C-L>
+endif
 
 set laststatus=2
 set ruler