Expose Axis tier intervals.

Right now there is no endpoint to manipulate possible time intervals for major and minor ticks.
With this PR I introduce an API endpoint to provide custom time intervals.
To cover basic scenario I provide function, which easily can filter existed interval within some range. It might go directly to previously mentioned API, which will accept array or desired range.

Close #1212.
If it's only for internal use, we should `_protect` it. Is it too troublesome to use `steps[0]`?