Add the standard template helpers

[RobLoach]

Allow Handlebars to use DocPad's Standard Template Helpers as #2.

Problem

<h2>getBlock blockname</h2>

{{{getBlock "scripts"}}}
{{{getBlock "styles"}}}
{{{getBlock "meta"}}}

It renders to:

<h2>getBlock blockname</h2>

[object Object]
[object Object]
[object Object]

Will we need a toHTML helper?

[balupton]

We may also need to do a bind as well:
https://github.com/docpad/docpad-plugin-jade/blob/master/src/jade.plugin.coffee#L75-L76

[balupton]

I have become too preoccupied by other things to review, merge, and release this PR.

However, you should have received an invite a while ago to join the DocPad Extras Team, which will give you write access to this repository, so you can merge in the PR.

For an orientation, or if you need any assistance following the semi-automatic release process from CONTRIBUTING.md, reach out via https://bevry.me/discord/dev