Add library of tick formatters

[chnn]

There are two issues with the current tick formatting API: #67, #79.

To fix these problems I propose we expose several tick formatting utilities in the vis library, which can handle common cases like formatting times or formatting quantities of bytes.

These utilities could be composed with user specific functions to fix #67.

It would also let us generate ticks more intelligently when the baked-in tick formatting utilities are used, since we can take into account the formatting when generating ticks. This would fix #79.