[MRG + 1] Create a new figure and test each plot type #127 (#179)

* [MRG] Create a new figure and test each plot type #127

 - move `plot()` to `plotting.py` as `plot_pdf()`
 - modify plotting functions to return matplotlib figures
 - add `test_plotting.py` and baseline images
 - import `plot_pdf()` in `__init__`
 - update `cli.py` to use `plot_pdf()`
 - update advanced usage docs to reflect changes

* Change matplotlib backend for image comparison tests

* Update plotting and tests
 - use matplotlib rectangle instead of `cv2.rectangle` in
`plot_contour()`
 - set matplotlib backend in `tests/__init__`
 - update contour plot baseline image
 - update `test_plotting` with more checks

* Update plot tests and config
 - remove unnecessary asserts
 - update setup.cfg and makefile with `--mpl`

* Add  to

* Add tolerance

* remove text from baseline plots
update plot tests with `remove_text`

* Change method name, update docs and add pep8

* Update docs

Makefile

@@ -15,7 +15,7 @@ install:
 	pip install ".[dev]"
 
 test:
-	pytest --verbose --cov-config .coveragerc --cov-report term --cov-report xml --cov=camelot tests
+	pytest --verbose --cov-config .coveragerc --cov-report term --cov-report xml --cov=camelot --mpl tests
 
 docs:
 	cd docs && make html
@@ -25,4 +25,4 @@ publish:
 	pip install twine
 	python setup.py sdist
 	twine upload dist/*
-	rm -fr build dist .egg camelot_py.egg-info
+	rm -fr build dist .egg camelot_py.egg-info