add support for blockParams

[fantapop]

The ability to define scoped variables was added in handlebars 3 however this needs to be added to each helper. It would be nice if the assemble-helpers supported this new functionality.

http://handlebarsjs.com/block_helpers.html#block-params

[assemblebot]

@fantapop 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.

[jonschlinkert]

Agreed, we'll get this implemented. thanks for the issue

[skysteve]

any progress on this? Just run into the same issue with the withGroup helper