Enhancement: ipfs files ln, for linking "files" to each other.

#### Type: Enhancement

#### Description:

So most operating systems have functionality that allows you to link together locations, so that modifying one modifies the other. I'm hoping that using the `files` subcommand, we could link together two separate files to always point to the same blocks.

Usage would be something like:

* `ipfs files ln new `, which will create a new file/folder at `` with the contents of ``, wherein changes to either one affect both. Preferably, one should be able to create multiple links to the same files.
* `ipfs files ln break ` will break the link between `` and any path it's currently linked to.

Neither command should actually change the hash of either object, of course.

Attempting to create a link where `` is a child of `` should be impossible, and vice-versa. If someone attempts to do something like `ipfs files mv /a /b/c` (where `/a` and `/b` are linked), if should be equivalent to breaking the link, before moving.