Lag/lead rename argument

[bearloga]

In #4 I suggested that the names of the horizon columns should be prefixed with mlts_lead_, so perhaps there should be some kind of argument that changes what the prefixes are:

• mlts_lag_ → mlts_previous_ or mlts_past_
• mlts_lead_ → mlts_next_ or mlts_future_

Totally optional argument that might make the output easier to read, especially to not accidentally mix up which columns you're using as the response(s) when training your model.

[bearloga]

prefix = c("technical", "friendly")???