Allow a specific configured program call to start languagetool:

On my distribution, archlinux, the pacman manager allow to install
languagetool. The package provide an unified script to start
languagetool command, server and gui. This update allow to use
vim-languagetool without using the jar file directly.

doc/LanguageTool.txt

@@ -171,6 +171,18 @@ g:languagetool_jar                                       *g:languagetool_jar*
 
   :let g:languagetool_jar='$HOME/languagetool/languagetool-standalone/target/LanguageTool-4.3-SNAPSHOT/LanguageTool-4.3-SNAPSHOT/languagetool-commandline.jar'
 
+g:languagetool_cmd                                       *g:languagetool_cmd*
+
+  Conversely, you could use this variable to specify by yourself the
+  command call. This variable take priority over g:languagetool_jar.
+  This is usefull for system package management installs like for
+  archlinux.
+
+  Example: >
+
+  :let g:languagetool_cmd='/usr/bin/languagetool'
+  :let g:languagetool_cmd='java -jar $HOME/languagetool/languagetool-standalone/target/LanguageTool-4.3-SNAPSHOT/LanguageTool-4.3-SNAPSHOT/languagetool-commandline.jar'
+
 g:languagetool_lang                                      *g:languagetool_lang*
 
   The language code to use for the language tool checker. If undefined,

plugin/LanguageTool.vim

@@ -176,7 +176,7 @@ function s:LanguageToolSetUp() "{{{1
   \ ? g:languagetool_jar
   \ : $HOME . '/languagetool/languagetool-commandline.jar'
 
-  if !filereadable(s:languagetool_jar)
+  if !exists("g:languagetool_cmd") && !filereadable(s:languagetool_jar)
     " Hmmm, can't find the jar file.  Try again with expand() in case user
     " set it up as: let g:languagetool_jar = '$HOME/languagetool-commandline.jar'
     let l:languagetool_jar = expand(s:languagetool_jar)
@@ -253,8 +253,11 @@ function s:LanguageToolCheck(line1, line2) "{{{1
   let l:range = a:line1 . ',' . a:line2
   silent exe l:range . 'w!' . l:tmpfilename
 
-  let l:languagetool_cmd = 'java'
-  \ . ' -jar '  . s:languagetool_jar
+  let l:languagetool_cmd = exists("g:languagetool_cmd")
+  \ ? g:languagetool_cmd
+  \ : 'java -jar ' . s:languagetool_jar
+
+  let l:languagetool_cmd = l:languagetool_cmd
   \ . ' -c '    . s:languagetool_encoding
   \ . (empty(s:languagetool_disable_rules) ? '' : ' -d '.s:languagetool_disable_rules)
   \ . (empty(s:languagetool_enable_rules) ?  '' : ' -e '.s:languagetool_enable_rules)