Add configuration option to suppress stderr (#80)

Closes #78

autoload/lsc/server.vim

@@ -256,7 +256,11 @@ function! lsc#server#register(filetype, config) abort
     call lsc#protocol#consumeMessage(self)
   endfunction
   function server.err_callback(message) abort
-    call lsc#message#error('StdErr from '.self.config.name.': '.a:message)
+    if self.status == 'starting'
+        \ || !has_key(self.config, 'suppress_stderr')
+        \ || !self.config.suppress_stderr
+      call lsc#message#error('StdErr from '.self.config.name.': '.a:message)
+    endif
   endfunction
   function server.on_exit() abort
     unlet self.channel

doc/lsc.txt

@@ -297,6 +297,12 @@ are configured with the same name use the same server.
 `'enabled'`: Set to `v:false` to avoid starting the server until
 |:LSClientEnable| is called.
 
+                                                *lsc-server-suppress_stderr*
+`'suppress_stderr'`: Set to `v:true` to suppress stderr output which is
+received after the server has been initialized. Stderr output before
+initialization still is shown in an error message. Use this if the server uses
+stderr for noisy status or logging.
+
                                                 *lsc-server-message_hooks*
 `'message_hooks'`: Set to a dictionary where the keys are LSP method names and
 the values are either |funcref|s which may modify '`params`' for the call, or