Added support for config files (#402)

* Partially reverted 5a3bf5f

* Added support for config files

Specify a config file with --config. Command line parameters have
precedence over the values read from the config file, as expected.

The config file is a json serialization of the namespace returned by
argparse, casted to a dictionary. The file args.json produced by the
training can be used as config file.

The config file does not need to be complete. Missing parameters will
be read from the command line or will take default values.

Additionally the functionality introduced in 5a3bf5f has been
reimplemented, accessing the argument_definitions member of
ConfigArgumentParser.

* Added unit tests for config files

* Addressed (most of) github comments

* switch from json to yaml

* changelog, minor version

* Changed config file format to YAML

* Typo

* mypi

* Added test overwriting config file argument with command line

CHANGELOG.md

@@ -10,6 +10,22 @@ Note that Sockeye has checks in place to not translate with an old model that wa
 
 Each version section may have have subsections for: _Added_, _Changed_, _Removed_, _Deprecated_, and _Fixed_.
 
+## [1.18.13]
+### Added
+- Added support for config files. Command line parameters have precedence over the values read from the config file.
+  Minimal working example:
+  `python -m sockeye.train --config config.yaml` with contents of `config.yaml` as follows: 
+  ```yaml
+  source: source.txt
+  target: target.txt
+  output: out
+  validation_source: valid.source.txt
+  validation_target: valid.target.txt
+  ```
+
+### Changed
+  The full set of arguments is serialized to `out/args.yaml` at the beginning of training (before json was used).
+
 ## [1.18.12]
 ### Changed
 - All source side sequences now get appended an additional end-of-sentence (EOS) symbol. This change is backwards

sockeye/__init__.py

@@ -11,4 +11,4 @@
 # express or implied. See the License for the specific language governing
 # permissions and limitations under the License.
 
-__version__ = '1.18.12'
+__version__ = '1.18.13'