Pass `meta` through to template engine.

Closes #6.
There are a few flies in the ointment here, I'm afraid: the first is that `consolidate` hasn't been updated for a while and pull requests are piling up; the second is that its tests are failing on Travis, so any new branch taken from `master` also fails, which is very annoying from a development point of view; and the third -- and most serious -- is that the author won't be updating the module anymore (you've probably already seen [Farewell Node.js](https://medium.com/code-adventures/farewell-node-js-4ba9e7f3e52b)).

Which means that although you are right that we should just wait for the upstream merge to happen...the problem is that we have no idea when or if that might happen!

Possible solutions are:

* leave the PR open until the merge happens, and I'll just refer to my fork in my projects that want to use `gulp-layoutize` with Liquid, possibly for ever. The problem with this is that I'll have to keep rebasing if you or anyone else enhances `gulp-layoutize`;
* offer to take over `consolidate` from TJ (he's looking for people to take over his modules), and then get the PR in. The problem with this is that taking on the module also means taking on responsibility for getting everyone else's PRs in;
* create a completely new module that does roughly the same thing as `consolidate` -- provide a facade to many different template engines -- but has a better design (something I was thinking of doing anyway). It would only need to have Jade and Liquid support to begin with to ensure that it could be used with `gulp-layoutize`.

What would be your preference @rowoot? Are you using `gulp-layoutize` regularly enough to want to see it evolve to support other templating libraries?