Update API to Julia Underscore Casing

[tawheeler]

The current API has a lot of methods that use camel case: tikzCode, plotHelper, etc.
This package was written before Julia settled on underscore case: tikz_code, plot_helper, etc.
Furthermore, some of these methods could be renamed to be verbs, such as get_tikz_code, plot_code, etc.

Unfortunately, these changes would probably not be backwards-compatible.