fixed bug in search endpoint

* when multiple accounts are registered, signal-cli expects the account
  specified with '-a'. In order to stay backwards compatible, the search
  endpoint was extended with an optional number parameter. i.e:
  '/search/{number}'. In case only one account is registered, the number
  parameter can be omitted.

see #369

src/docs/docs.go

@@ -1289,6 +1289,12 @@ var doc = `{
                 ],
                 "summary": "Check if one or more phone numbers are registered with the Signal Service.",
                 "parameters": [
+                    {
+                        "type": "string",
+                        "description": "Registered Phone Number",
+                        "name": "number",
+                        "in": "path"
+                    },
                     {
                         "type": "array",
                         "items": {