Add tick formatter plot options

[philippjfr]

Adds plot options to control the axis formatting directly rather than using the Dimension.value_format which only accepts formatter functions. These accept:

• Format strings like '%d', '%.3f' etc.
• Python functions which are compiled to JS with flexx (or converted to a FuncFormatter in mpl)
• TickFormatter instances (or matplotlib.ticker.Formatter instances for mpl)

• Add unit tests
• Add documentation (to styling plots notebook)

[philippjfr]

@jlstevens Ready to review and merge once tests are passing.

[jlstevens]

Looks good!

I feel that maybe we should have used bokeh's PrintfTickFormatter earlier but better late than never! I'll merge once the push build is green.

[philippjfr]

I feel that maybe we should have used bokeh's PrintfTickFormatter earlier but better late than never!

Definitely, I've never been able to give a good answer to questions about tick formatting.

[jlstevens]

Hmm, one thing...shouldn't matplotlib match the plot options here? (if possible)

[philippjfr]

Hmm, one thing...shouldn't matplotlib match the plot options here?

Aren't they matching?