Add `options.partialsdir` to cwd partials

I plan to use Assemble to create a pattern library, using Brad Frost's atomic design principles as a foundation. Put simply, I just need to change the way partials are stored, so that we can call them as follows:

{{> directory-filename}}

where in fact, the partial is physically stored in

directory/(subdirectory/)*filename.hbs

In order to do that, I need to update the way partials are registered in Assemble, as follows:

(1) Add an option partialsdir akin to options.layoutdir
(2) Update assemblePartials to store partials into assemble.options.data[directory '-' + filename]

And that's all, hopefully.