Fix RmdOperator to not produce a folder with assets in dag directory

Currently, it uses the default behavior of creating a folder named _files. With gusty v0.3+ this results in a arbitrary task group that doesn't do anything. If we add a `_` or `.` before the folder name it should fix this.

Another challenge is that if a person knits the file manually it will result in the original folder name, so I would hold off on upgrading gusty until we can get a sense for how people run / develop the Rmarkdown files.