Expose shouldNice as an axis configuration option

[Brodzko]

Currently there is not an option to not nice the axes other than passing explicit min/max/hardMin/hardMax. Sometimes it is useful to just use the data range and not nice the axes by default.

Unless I missed something it would just require changing this and this to default to a provided option value. Disabling nicing in cases of explicit extrema is good either way.

[Brodzko]

Also in buildTimeAxis the checks for extrema should be checking for Dates, not numbers :)

[Brodzko]

Resolved in #235