Support composer.bat on Windows

Fixes #11

Author: noahfrederick

autoload/composer.vim

@@ -232,6 +232,8 @@ endfunction
 function! s:project_makeprg() dict abort
   if self.has_file('composer.phar')
     return 'php composer.phar'
+  elseif executable('composer.bat')
+    return 'composer.bat'
   else
     return 'composer'
   endif

t/project.vim

@@ -45,7 +45,7 @@ describe 'composer#project()'
 
     context 's:project_makeprg()'
       it 'returns the composer executable name'
-        Expect b:project.makeprg() ==# 'composer'
+        Expect b:project.makeprg() =~# '^composer'
       end
     end
   end