add functionality to make it easy to change target output locations

[jsadler2]

For the use case where we are working on Tallgrass/Caldera, it would be helpful to make the location where the targets are built easily changeable. That way we can have a central data store on a shared folder in Caldera. In addition to it being shared (which will be nice so that targets don't have to be rebuilt unnecessarily), it's better to have the actual data in Caldera as opposed to our home directories.

[jsadler2]

This is easy to do for the non-file targets - you add a _targets.yaml file and change the "store" variable. It will be a little trickier for the file targets. I think what I'll do is just have some string manipulation in the _targets.R file to create the output dir variable and that will be concatenated with the output file directory paths.