Add directory field to context

* Cookiecutter version: 1.7.2
* Template project url:
* Python version: 3.7.7
* Operating System: Mac OS X

### Description:

I would like the context output by `{{cookiecutter | jsonify}}` to include the `-directory` argument passed to the CLI, under a '_directory' field. This will allow tooling created to update projects with new versions of their base templates to correctly pick that template.

### What I've run:

`{{cookiecutter | jsonify}}`, which when run against a template in a git repo, with the '-directory' argument, has the `_template` field as e.g. git+https://bitbucket.org/{org}/{repo}, which is not enough information to resolve the template used if it was in a subdirectory of the repo.

Trace how the `-directory` CLI argument is handled and how `{{cookiecutter | jsonify}}` produces the context. Done means the context includes a `_directory` field so tooling can identify templates stored in repository subdirectories.

Written by the indexing model from the issue text.