feat: support headless mode (#199)

Co-authored-by: Subhaditya Nath <sn03.general@gmail.com>

README.md

@@ -307,7 +307,21 @@ let g:doge_python_settings = {
 
 - `single_quotes`: Whether or not to use single quotes for the multi-line comments openers and closers
 
-# FAQ
+# Headless mode
+
+If you're running your vim commands inside a docker, CI or similar environments
+with commands such as `vim +PlugInstall +qall > /dev/null` then you probably
+want to use headless mode. This will not spawn any terminals, progress bars etc
+and will simply run any process by vim-doge in the background.
+
+This feature can be enabled by passing in `{ 'headless': 1 }` into the
+`doge#install()` like so: `doge#install({ 'headless': 1 })`.
+
+Example using vim-plug:
+
+```vim
+Plug 'kkoomen/vim-doge', {'do': { ->doge#install({ 'headless': 1 }) }}
+```
 
 # Help
 

autoload/doge.vim

@@ -180,7 +180,7 @@ endfunction
 
 "" @public
 " Install the necessary dependencies.
-function! doge#install() abort
+function! doge#install(...) abort
   for l:filename in ['vim-doge', 'vim-doge.exe']
     let l:filepath = g:doge_dir . '/bin/' . l:filename
     if filereadable(l:filepath)
@@ -212,6 +212,14 @@ function! doge#install() abort
     let l:term_height = 4
   endif
 
+  if len(a:000) > 0
+    if get(a:000[0], 'headless', 0)
+      call system(l:command)
+      call s:report_result(v:shell_error)
+      return
+    endif
+  endif
+
   if has('nvim') && exists(':terminal') == 2
     " neovim with :terminal support
     " vint: next-line -ProhibitUnusedVariable