Add test coverage and docs to CI

tox.ini

 [tox]
-envlist = py36, py37, flake8
+envlist = py36, py37, flake8, docs, coverage
 
 [travis]
 python =
@@ -11,6 +11,16 @@ basepython = python
 deps = flake8
 commands = flake8 flatland
 
+[testenv:docs]
+basepython = python
+whitelist_externals = make
+commands = make docs
+
+[testenv:coverage]
+basepython = python
+whitelist_externals = make
+commands = make coverage
+
 [testenv]
 setenv =
     PYTHONPATH = {toxinidir}