Altair-saver

- Fix bug when running as root user on linux (59)
- node-based saver hides window warnings that were displayed when saving interactive charts
(53, 57)
- node-based saver now accepts ``vega_cli_options`` that will be passed to command line (52)

- ``altair_saver.types`` is now a public module. (47)
- ``altair_saver.JavascriptError`` now importable at top-level. (46)
- Added top-level ``available_formats()`` function, which returns the set of
available formats. (43)

- Fix bug in detecting npm binary path (42)

Behavior changes

- ``save()`` now returns the serialized chart if ``fp`` is not specified (41).
- ``fmt="json"`` now saves the input spec directly for both vega and vega-lite input.
Additionally, the ``json`` format in ``render()`` outputs a JSON mimetype rather than
a vega-lite mimetype (34).
- ``render()`` and ``save()`` with HTML format now have a ``standalone`` argument
that defaults to True for ``save()`` and False for ``render()``, so that HTML
output will work better in a variety of notebook frontends (33).
- HTML and Selenium output now respects embedding options set via
``alt.renderers.set_embed_options`` (30, 31).

Maintenance
- much improved documentation & test coverage.

Behavior changes
- selenium: prefer chromedriver over geckodriver when both are available (27)

Bug Fixes
- selenium: altair_saver respects altair themes (22)
- selenium: improve javascript-side error handling (19)