Make startify#fortune#boxed() take optional argument

References #285.

autoload/startify/fortune.vim

@@ -157,9 +157,9 @@ function! startify#fortune#quote() abort
 endfunction
 
 " Function: #boxed {{{1
-function! startify#fortune#boxed() abort
+function! startify#fortune#boxed(...) abort
   let wrapped_quote = []
-  let quote = startify#fortune#quote()
+  let quote = a:0 && type(a:1) == type([]) ? a:1 : startify#fortune#quote()
   for line in quote
     let wrapped_quote += split(line, '\%50c.\{-}\zs\s', 1)
   endfor

doc/startify.txt

@@ -542,9 +542,9 @@ Dynamic example:~
 <
 If you go for a dynamic header, you might find the following functions useful:
 
-    startify#fortune#quote()    raw random quote
-    startify#fortune#boxed()    formatted random quote in a box
-    startify#fortune#cowsay()   formatted random quote in a box + cow
+    startify#fortune#quote()     raw random quote
+    startify#fortune#boxed(...)  opt list or formatted random quote in a box
+    startify#fortune#cowsay()    formatted random quote in a box + cow
 
 Try them like this:
 >