Custom arguments in g:mkdp_browserfunc and :MarkdownPreview

Hi

I was thinking passing custom arguments to browser function, something like:

```viml
function! g:Mkdp_custombrowser(url, custom_args)
silent exec "chromium " . a:custom_args . " --app=" . a:url . " &"
endfunc

let g:mkdp_browserfunc = 'g:Mkdp_custombrowser'
```

And call it like:

`:MarkdownPreview "--arg1=\"foo\" --arg2=\"bar\""`

I was trying to hack around the codebase with something like:

```viml
" plugin/mkdp.vim:
function! s:MkdpAU() abort
command! -buffer -nargs=* MarkdownPreview call mkdp#util#open_preview_page()
call s:init_command()
endfunction

" autoload/mkdp/util.vim:
" open preview page
function! mkdp#util#open_preview_page(args) abort
if get(s:, 'try_id', '') !=# ''
return
endif
let l:server_status = mkdp#rpc#get_server_status()
if l:server_status ==# -1
call mkdp#rpc#start_server()
elseif l:server_status ==# 0
let s:try_id = timer_start(1000, function('s:try_open_preview_page'))
else
call mkdp#util#open_browser(a:args)
endif
endfunction

" open browser
function! mkdp#util#open_browser(args) abort
call mkdp#rpc#open_browser()
call mkdp#autocmd#init()
call mkdp#command#init()
endfunction
```

But got:

```viml
:MarkdownPreview foo

[markdown-preview.vim] UnhandledRejection
Error: nvim_call_function: Vim:E119: Not enough arguments for function: mkdp#util#open_browser
```

I'm not very well versed in VimScript.

Thanks!