Use gflags to parse command line arguments for convert_imageset

In the dev branch, many new options have been added to the utility convert_imageset, which makes the old style parsing tedious and the usage hard to remember. This PR aims to replace positional argument parsing with `gflags`, now that it becomes a dependency.
Instead of keeping some of the names still hard-coded, you can use gflags for them too. It would make the command longer but is usually acceptable.