Extend 1D plotting capabilities. Resolves #581.

### Extended 1D plotting capabilities

This PR changes the 1D plotting interfaces (`iris.plot.plot` and `iris.quickplot.plot`) so that they take either one or two positional arguments (like `matplotlib.pyploy.plot`) which can be either coordinates or cubes. This allows the user to have full control over the resulting plot.

This replaces the much more limited #582 (hat tip to @esc24: you were right, I was wrong... this is a much better idea!)

#### Summary

`iris.plot.plot` and `iris.quickplot.plot` can take up to two positional arguments, which can be either coordinates or cubes. The `coords` keyword argument has been deprecated in these functions as it is no longer necessary/useful.

* Deprecated coords keyword in `iris.plot.plot`
* Re-written `_draw_1d_from_points` to handle one or two cubes or coordinates, done in such a way that #522 and other future additions can be made with minimal effort.
* Deprecated coords keyword in `iris.quickplot.plot`.
* New labelling function for quickplot as the existing code could not easily be bent to handle the new feature.
* Removed `coords` keyword argument from examples that use it for 1D plots.
* Added tests for the new behaviour.

#### Remaining issues

* Decide what (if anything) should be the title for plots of two coordinates or two cubes when using the quickplot interface.
* Should coord vs. coord *maps* have axis labels? Currently they do not which is consistent with the other map plotting functions.