Multipackage

- Refactor code to unify all error and warning messages into a generic message
structure with iter_messages(type) to select warnings or errors.
- Allow templates to declare what environment variables they need based on their
configured settings.
- Implement `multipackage doctor` to show what environment variables are
optional or required for a given repository.

- Update `ManagedFileSection.ensure_lines` to support matching multiple lines
with regular expressions so that you can do things like changing the pinned
version of a given package in a `requirements.txt` file.
- Update `atomic_json` and `render_template` to produce files your native
line ending by default so that the files are more easily readable on each
platform (since git checks them out in native line endings anyway).
- Remove Sphinx warnings from `pytest` since they are not fixable and we can't
move to Sphinx 2.0 anyway because it doesn't support Python 2.7.
- Move to a template based approach for repository management where each repo
has a template that installs and configures subsystems. These subsystems are
then in charge of managing the repository. There is one default template
named `pypi_package` that sets up a repository for releasing pypi packages.
- Add `test_by_name.py` script to handle entering each component subdirectory
and running the required test commands.

- Update location of multipackage files to all live under a `.multipackage`
directory. This consolidates everything into a single place and avoids
cluttering up the repository. It makes `multipackage` more like `git` in that
it has a folder that it manages for you and a cli tool that interacts with
that folder.

- Initial commit