Improve API to be in line with constructor

[aendra-rininsland]

This is a meta issue intended to collect all the features currently provided by instantiating C3 via constructor that currently aren't available via the API.

(That is, "you can do something by calling c3.generate(), but then can't modify that thing via chart.whatever()").

FR = "First Reported"

• Change x-axis type (FR Provide a way to programatically change x axis type (ex. timeseries, category, index) #1456)
• Set axis tick count/culling/format properties (FR @megapctr here)

[missingdays]

#1760 for showing and hidding axis and #1752 for chart.width() and chart.height() I think is relevant to this