Add --location option to add for ad-hoc dependencies

[njlr]

Currently the user must edit the TOML manually.

This is OK, but we should also support this in the CLI.

For example, this command:

buckaroo add owner/project@branch=master --location git@url/to/git/repo.git

Would generate this TOML:

[[location]]
package = "owner/project"
git = "git@url/to/git/repo.git"

[[dependency]]
package = "owner/project"
version = "branch=master"

The exact syntax is still up for grabs.