add root argument

[gaborcsardi]

No description provided.

[gaborcsardi]

Closed by #433.

Although I might need to change the semantics because it does not seem to make sense to have a root if some paths are not included in it. It also does not seem to make sense to have NA_character_ for root, because renv also read the ignore files from the subdirectories.

Maybe if root = NA_character_ then we don't read any ignore files and treat path as individual files, ignoring their path, essentially.

In general this is way too complicated for use cases that rarely ever happen, so I am not happy with it at all.