add comamnd should use project manager POST endpoint

[tlugger]

When a nio add <block> is run, the cli manually creates the submodule in the blocks/ directory, installs dependencies from requirements.txt, and updates etc/blocks.cfg with the new block data.

The project manager component in nio currently opens up an endpoint where one could POST to /project/blocks with data like:

 {
        "branch": null,
        "path": null,
        "tag": null,
        "url": "git://github.com/nio-blocks/<blockType>.git"
}

Which allows nio download the new block and its dependencies without manually changing the files running on the machine.