Implement a maxgap option

[rob-ruz]

It would be greeat if there was an option that let you specify the maximum number of consecutive NA's to fill, like in other interpolation functions from packages like zoo and imputeTS.

I 'm working with time series of monthly stock prices and I would like to impute a max of 12 consecutive missing values with impute_AR1_t and left unchanged larger gaps.

Thank you for your work on this package, it is really great.