Enable CLI to accept a directory of files as the filepath argument

[lmckone]

It would helpful if the CLI allowed users to specify a directory of files to be uploaded, similar to how the java implementation of DVUploader accepts a directory name in place of the filepath via directoryname in place of file.csv in the command

[anarchivist]

i've been looking at this for work originally as a separate script for my own use, but increasingly i'm interested in possibly adding this to the CLI myself as a --recurse/--no-recurse option. a couple questions on implementation:

• is it right to assume that this a top-level configuration option? (in other words, this is not something that we expect to propagate to each level down in the files entries in a YAML config file.)
• i'm assuming that would want logic to determine if the the filepath is a file or directory, and then either call dvuploader.utils.add_directory() or construct a new instance of dvuploader.File. it could be simpler to assume that that the presence of --recurse means that it should only expect a directory, but that feels more brittle.

thoughts welcome.

[anarchivist]

as shown, i've added PR #28, using the top-level configuration only approach. happy to get feedback.