Use filterText for filtering candidates if available.

Per spec filterText should be used for filtering when the server provides it.
If it's not provided, label should be used.

Some language server such as clangd adds some prefix to labels, such as empty
spaces. They don't work well with prefix-based filter since the additional prefix
will never match the completion prefix.

Should fix #92 and may also help #79.

company-lsp.el

@@ -387,15 +387,27 @@ Return a list of strings as the completion candidates."
       candidates)))
 
 (defun company-lsp--filter-candidates (candidates prefix)
-  "Filters CANDIDATES by PREFIX.
+  "Filter CANDIDATES by PREFIX.
 
 CANDIDATES are a list of strings of candidate labels created by
 `company-lsp--make-candidate'.
 
 Returns a new list of candidates."
   ;; TODO: Allow customizing matching functions to support fuzzy matching.
   ;; Consider supporting company-flx out of box.
-  (all-completions prefix candidates))
+  (-filter (lambda (candidate)
+             (s-starts-with-p prefix (company-lsp--candidate-filter-text candidate) t))
+           candidates))
+
+(defun company-lsp--candidate-filter-text (candidate)
+  "Return filter string of CANDIDATE.
+
+CANDIDATE is a string created by `company-lsp--make-candidate'.
+If the CompletionItem of CANDIDATE has filterText field, return
+the value of filterText. Otherwise return CANDIDATE itself."
+  (let* ((candidate-item (company-lsp--candidate-item candidate))
+         (filter-text (gethash "filterText" candidate-item)))
+    (or filter-text candidate)))
 
 (defun company-lsp--cleanup-cache (_)
   "Clean up completion cache and company hooks."