New CLI command 'add', better Windows support, fixes

This release is made possible by several contributions from @sirikon, @olliephillips, @rohanpai

#78
adds better Windows support & fixes an issue with the CLI

#87
includes a new CLI command $ponzu add <import path> to go get & copy addon packages into a Ponzu project

Additionally, there was a bug in the internal omit func called during the contentsHandler of the content API. This is now fixed, and we advise everyone to upgrade your projects to v0.8.5 after updating your ponzu package.

To get the latest release:

$ go get -u github.com/ponzu-cms/ponzu/...
$ cd $PROJ
$ ponzu upgrade