Standard plotting configuration

This pull request will provide a standard plotting configuration for astropy.  The new `set_astropy_plot_style()` function sets matplotlibrc parameters to reflect the style used in `astropy/astropy-tutorials/templates/matplotlibrc`.  This can be used in tutorials as such:

```python
from astropy.utils.plotting import set_astropy_plot_style
set_astropy_plot_style()
```

This pull request is in regards to issue #2719.