Duplicated syntax formatters when adding same syntax

[jcubic]

Issue summary

When you call same syntax multiple times, terminal created duplicated formatters

Expected behavior

It should remove old syntax formatter and create new one or ignore when there is one already

Actual behavior

There are duplicated formatters

Steps to reproduce

execute:

$.terminal.syntax("scheme");
$.terminal.syntax("scheme");
$.terminal.syntax("scheme");

there will be 3 scheme formatters.