Allow a context to contain its own template.

Hi there.

I had my own module which wrapped `url-template` in order to allow me to create config settings that combine settings and a URI template.

I was just about to push it to NPM so that I can use it in a couple of other projects when I realised that if I were to add this one statement to `url-template` I wouldn't need any wrapper code at all.

...hence the pull request.

Thanks for a very useful module, by the way.

Mark