Add option to set custom templateParameters

[Brondahl]

Add ability to define custom TemplateParameters. Useful, for example, for injecting C# constant strings into the JS files that we want to c.InjectJavascript() into the Swagger page.

I've already tested that, for example, referencing %(DocumentTitle) in my injected Javascript file works, but I can't do that with my own template key, making the isTemplate parameter of InjectJavascript() pretty pointless.