Consistent way to access `view` inside Nunjucks Tag

[dtothefp]

As discussed here https://github.com/assemble/assemble-core/issues/13 and here https://github.com/jonschlinkert/templates/issues/17

view on the file context is difficult in Nunjucks. A typical nunjucks tag can be constructed like so https://mozilla.github.io/nunjucks/api.html#custom-tags and you can see an example in my assemble sample repo https://github.com/dtothefp/assemble-view-issue/blob/master/tags/debug.js

I was previously accessing view off of context.ctx in the Nunjucks tag, and you suggested I can access it in a helper using this.view but I don't know how this transfers into Nunjucks land

[assemblebot]

@dtothefp Thanks for the issue! If you're reporting a bug, please be sure to include:

• The version of assemble you are using.
• Your assemblefile.js (This can be in a gist)
• The commandline output. (Screenshot or gist is fine)
• What you expected to happen instead.