Dictionaries are prebuilt and saved as part of the data object.
The config decides whether or not to use the dictionary if it is
available. Configs that require dictionaries are only run with
data that have dictionaries. The method will skip configs that are
irrelevant, so for example ZSTD_compress() will skip configs with
dictionaries.

I've also trimmed the silesia source to 1MB per file (12 MB total),
and added 500 samples from the github data set with a dictionary.

I've intentionally added an extra line to the `results.csv` to make
the nightly build fail, so that we can see how CircleCI reports it.

Full list of changes:

* Add pre-built dictionaries to the data.
* Add `use_dictionary` and `no_pledged_src_size` flags to the config.
* Add a config using a dictionary for every level.
* Add a config that specifies no pledged source size.
* Support dictionaries and streaming in the `zstdcli` method.
* Add a context-reuse method using `ZSTD_compressCCtx()`.
* Clean up the formatting of the `results.csv` file to align columns.
* Add `--data`, `--config`, and `--method` flags to constrain each
  to a particular value. This is useful for debugging a failure
  or debugging a particular config/method/data.

The regression tests run nightly or on the `regression`
branch for convenience. The results get uploaded as the
artifacts of the job. If they change, check the diff
printed in the job. If all is well, download the new
results and commit them to the repo.

This code will only run on a UNIX like platform. It
could be made to run on Windows, but I don't think that
it is necessary. It also uses C99.

* data: This module defines the data to run tests on.
  It downloads data from a URL into a cache directory,
  checks it against a checksum, and unpacks it. It also
  provides helpers for accessing the data.
* config: This module defines the configs to run tests
  with. A config is a set of API parameters and a set of
  CLI flags.
* result: This module is a helper for method that defines
  the result type.
* method: This module defines the compression methods
  to test. It is what runs the regression test using the
  data and the config. It reports the total compressed
  size, or an error/skip.
* test: This is the test binary that runs the tests for
  every (data, config, method) tuple, and prints the
  results to the output file and stderr.
* results.csv: The results that the current commit is
  expected to produce.