Add support of overriding languageclient path

Close #1020

Author: autozimu

autoload/LanguageClient.vim

@@ -641,6 +641,11 @@ endfunction
 
 let s:root = expand('<sfile>:p:h:h')
 function! LanguageClient#binaryPath() abort
+    let l:path = s:GetVar('LanguageClient_binaryPath')
+    if l:path isnot v:null
+        return l:path
+    endif
+
     let l:filename = 'languageclient'
     if has('win32')
         let l:filename .= '.exe'

doc/LanguageClient.txt

@@ -596,6 +596,13 @@ Whether set buffer omnifunc to 'LanguageClient#complete'.
 Default: v:true
 Valid options: v:true | v:false
 
+2.39 g:LanguageClient_binaryPath                  *g:LanguageClient_binaryPath*
+
+Full path to languageclient binary.
+
+Default: 'bin/languageclient' relative to plugin root.
+Valid options: String
+
 ==============================================================================
 3. Commands                                           *LanguageClientCommands*