Adding dynamic plugin

Dynamic plugin. Only calculating `assets` right now.
Test for dynamic
Modifications to dest and assets handling

Updating dynamic plugin to make an `assets` computed property so assets can be recalculated when `dest.dirname` changes.

Updating `assemble.dest` to wrap the `dest` argument in a function to return the correct file dest.

Adding `dest-path` plugin to update `file.dest` properties based on the `dest` passed into `assemble.dest`.