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.
(see above - if we use all gflags, we won't need to set a custom help, which may be simpler.)