This changes it so that it is an error if there is ever an unknown
configuration field. This is intended to help avoid things like typos,
or using an outdated version of mdbook. Although it is possible that new
fields could potentially safely be ignored, setting up a warning system
is a bit more of a hassle. I don't think mdbook needs to have the same
kind of multi-version support as something like cargo does. However, if
this ends up being too much of a pain point, we can try to add a warning
system instead.
There are a variety of changes here:
- The top-level config namespace is now closed so that it only accepts
the keys defined in `Config`.
- All config tables now reject unknown fields.
- Added `Config::outputs` and `Config::preprocessors` for convenience
to access the entire `output` and `preprocessor` tables.
- Moved the unit-tests that were setting environment variables to the
testsuite where it launches a process instead.
Closes https://github.com/rust-lang/mdBook/issues/1595
