New feature: micrositeExtraDirectory

[calvellido]

As part of the issue frees-io/freestyle#454 I found out that it could be interesting to have a feature in sbt-microsites where a directory can be set to the be copied to the jekyll processed target generated directory structure, but without adding any additional process to it, just a copy.

In the mentioned issue it would be of help as it could let adding to the project some JS files needed for the home layout but not for the rest of the docs pages.

The goal would be to have a SBT setting in the same vein as the current ones, named micrositeExtraDirectory, that wouldn't be set by default.

Any thoughts? Do you think that this would be ok?

[calvellido]

As Jekyll call them static files, that could be a better name for it: micrositeStaticDirectory

https://jekyllrb.com/docs/static-files/