allow graphite metrics name contain '~'

when a metrics name contains '~', id does not impact graph display.
but you can not use grafana UI to edit metrics with realtime
graphite query.

public/app/plugins/datasource/graphite/lexer.js

@@ -119,6 +119,7 @@ define([
     identifierStartTable[i] =
       i >= 48 && i <= 57 || // 0-9
       i === 36 ||           // $
+      i === 126 ||          // ~
       i >= 65 && i <= 90 || // A-Z
       i === 95 ||           // _
       i === 45 ||           // -