Lander

- New plugin framework to customize how Lander retrieves metadata from LaTeX documents, and their Git repositories and CI/build environments.
You can create a new plugin by creating a package that registers a subclass of `lander.ext.parser.Parser` with the `lander.parser` setuptools entry point.

- Update Jinja to >=2.10.1 to address CVE-2019-10906.
- Update asset build pipeline to Gulp 4 and update all other npm dependencies at the same time.

- Update to `lsst-projectmeta-kit` to 0.3.5 for better author parsing.
- Update `requests` to 2.20.0 (security).

- Lander will now cleanly abort when building on Travis CI, but the secure environment variables are not available.
This happens if the build is triggered by a fork.
- Switched to use `$TRAVIS_BUILD_WEB_URL` to get a URL to the build.
This is better than computing the build URL during the travis-ci.org to travis-ci.com transition.
- Updated test dependencies to pytest 4.2.1.
- Switched to `setuptools_scm` for version string management.

- Update to lsst-projectmeta-kit 0.3.3 for improved detection of `\input` and `\include` commands in TeX source.

- Update to `lsst-projectmeta-kit` 0.3.1 for more reliable `metadata.jsonld` generation (works around pandoc issues converting some documents to plain text).
There's a new integration test `make dmtn070` that demos this.
- Improve the testing strategy:
- Run `make pytest` to run pytest with the correct arguments instead of using `--add-opts` in setup.cfg. This lets us run `pytest` directly with ad hoc arguments.
- Run `make test` to run both pytest and the integration tests.