Rethink the display mechanisms #269

Description of the desired feature

Currently, there many ways of displaying a PyGMT figure:

• Calling Figure.show() in the notebook returns an IPython PNG display object for the jupyter notebook
• Putting the Figure object at the end of a notebook cell displays the figure as PNG
• Calling Figure.show(external=True) will open a preview in an external PDF viewer or the browser. The external viewer is non-blocking so this doesn't work for a script.
• Calling Figure.savefig(..., show=True) will save the figure and open it on an external viewer

This is not ideal. It would be better to have:

• Calling Figure.show() opens a preview of the figure and halts the Python process until the viewer is closed. It can also return self so that if it's called at the end of a notebook cell it would get displayed on the notebook as well.
• Putting the Figure at the end of a notebook cell displays a PNG preview.
• Having some way of tell Figure.show() to not open the viewer. This is required when using a notebook or when building the docs (we don't want windows opening when we build the gallery or tutorials). This can be done through a function pygmt.enable_notebook which would set a flag to tell show not to do anything and/or through an environment variable PYGMT_DISPLAY which can be set to none or other options that the user might want. Ideally, we should have both. The env variable will be useful when building the docs.