Add option to add arbitrary metadata to GridFS file during upload

Currently in mongofiles, the only form of metadata we allow for a GridFS file is with the -t / --type option. This specifies the type of the file, but there's no mechanism to actually specify any metadata that would have that type.

The GridFS files.contentType field is also deprecated, so we might want to consider removing the --type option altogether and replacing it with a --metadata option.