Support select by nearest point to mouse

[squaregoldfish]

This pull request implements a new selectMode option that allows switching the point selection mode between the closest X axis value (current behaviour) and the closest point to the mouse.

The option is activated by setting selectMode to either closest-x (the default current behaviour) or euclidian.

The changes that locked series, highlighted series and stacked graphs make to the selection behaviour have been carried across to the new euclidian mode to mimic existing behaviour as closely as possible. Tests have been added for all combinations of these options.

This work is related to issue #371. An additional change to getSelection() is required to handle multiple points with the same X axis value (#914), which is also included here.

There are two open questions:

1. I have put the documentation for selectMode under Interactive Elements. If you want it in another section, let me know.
2. I'm using findStackedPoint as it is currently written. However we now only use the series name, and none of the other information it returns. Should I cut down the method to only return a series name (and rename it to findStackedSeries), or leave it as it is in case it's useful in the future?

[coveralls]

Coverage increased (+0.1%) to 90.298% when pulling 49aad3a9c2bfbddbd1aebb4e9f9ce35626de603c on squaregoldfish:nearest-point-opt into 6611837c5490f2f4b6d61967aa1ad658ed5f11d4 on danvk:master.