feature: new view type "blocks"

[jonschlinkert]

I'd like to propose that we add the blocks view type, along with preBlocks and postBlocks middleware handlers to make this part of the render cycle.

Similar to layouts, "blocks" are engine-agnostic and can be applied before rendering using dry.

thoughts? questions?

[assemblebot]

@jonschlinkert 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.

If your issue is related to one of the following, please open an issue there:

• grunt-assemble Issues with using assemble in grunt or the grunt-assemble library.
• handlebars-helpers Issues with using handlebars helpers from the handlebars-helpers library.