Remove hard-coded dependency to requireJS Text plugin

Hello,

as I find the common "text" plugin name a bit too generic, I usually create a dependency named "require.text". As such, loading of the external templates would fail, trying to load the plugin as "text.js". 

This fix allows to define the Text plugin name, as:

    ko.amdTemplateEngine.defaultRequireTextPluginName = "require.text";