Download binary regardless cargo existed or not

[ray-x]

Is your feature request related to a problem? Please describe.
Currently, clap takes too much disk space because build maple cost me around 460MB.

function! clap#installer#install(try_download) abort
  " Always prefer to compile it locally.
  if executable('cargo')
    call clap#installer#build_all()
 

The code shows that if cargo installed, there is no option to download the prebuild.

Describe the solution you'd like
An option to download package directly. Specially for the user have cargo installed.

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[liuchengxu]

Use:

Plug 'liuchengxu/vim-clap', { 'do': { -> clap#installer#force_download() } }