add custom offset argument to `model_income_tax()`

[MattCowgill]

Politicians have recently taken to proposing new offsets (LMITO, WATR). This trend may continue.

Our current approach to incorporating a new offset into model_income_tax() is to have a logical argument, such as Budget2018_lamington or Budget2019_watr. This is fine, but it means that considering a new or amended offset (other than the LITO) necessitates updating the model_income_tax() function itself.

We should add a new custom_offset argument to model_income_tax() that operates like lito_multi - the user can specify the relevant intercept and phase in/out rates and thresholds. This will make the function more flexible and more able to consider alternative policies on the fly.

[MattCowgill]

planning to do this over summer