Graphtransliterator

------------------
* flake8 fix for core.py
* fixed bug in schemas.py whereby, during load(), DirectedGraphSchema() was modifying
input settings
* added tests for modifications to settings by load()
* adjusted DirectedGraphSchema to allow for compacted transliteration rule settings
* adjusted GraphTransliteratorSchema to allow for compacted settings
* added tests to confirm all optional fields passed to load() are really optional
* added ValidationError if onmatch_rules_lookup present without onmatch_rules
* adjusted DirectedGraphSchema edge definition to remove str if loading from JSON
* added more rigorous schema definitions for edge_list and node in DirectedGraphSchema
* fixed flake8 warning in graphs.py
* adjusted docstrings in core.py for dump(), dumps(), load(), and loads()

------------------
* added sphinx-issues and settings to requirements_dev.txt, docs/conf.py
* added .readthedocs.yml configuration file to accommodate sphinx-issues
* removed history from setup.py due to sphinx-issues
* fixed GraphTransliteratorException import in __init__.py
* added docs/_static directory
* fixed emphasis error and duplicate object description in docs/usages.rst
* fixed docstring in core.py
* added python versions badge to README.rst (:issue:`openjournals/joss-reviews1717`).
Thanks :user:`vc1492a`.
* added NOTICE listing licenses of open-source text and code
* added Dependencies information to docs/install.rst
(:issue:`openjournals/joss-reviews1717`). Thanks :user:`vc1492a`.
* updated AUTHORS.rst
* minor updates to README.rst

------------------
* fixed missing marshmallow dependency (:pr:`47`). Thanks :user:`vc1492a`.
* removed unused code from test (:pr:`47`). Thanks :user:`vc1492a`.
* removed cerberus dependency

------------------
* fixed error in README.rst

------------------
* adjustments to README.rst
* cleanup in initialize.py and core.py
* fix to docs/api.rst
* adjusted setup.cfg for bumpversion of core.py
* adjusted requirements.txt
* removed note about namedtuple in dump docs
* adjusted docs (api.rst, etc.)

-------------------
* Removed _tokens_of() from init
* Removed serialize()
* Added load() to GraphTransliterator, without ambiguity checking
* Added dump() and dumps() to GraphTransliterator to export configuration
* renamed _tokenizer_from() to _tokenizer_pattern_from(), and so that regex is compiled
on load and passed as pattern string (tokenizer_pattern)
* added settings parameters to DirectedGraph
* added OnMatchRule as namedtuple for consistency
* added new GraphTransliterator.from_dict(), which validates from_yaml()
* renamed GraphTransliterator.from_dict() to GraphTransliterator.from_easyreading_dict()
* added schemas.py
* removed validate.py
* removed cerberus and added marshmallow to validate.py
* adjusted tests
* Removed check_settings parameter