[h5diff] add abilty to exclude specific attributes

This is a request to extend or add a feature to h5diff to exclude specific attributes. Currently it can only exclude from comparison all the attributes attached to an object.

The background to this is that we use h5diff for the verification stage of our regression testing. We compare the HDF5 file produced to a 'known good' file using h5diff.

Some objects whose value is specific to the actual run (such as execution date/time) will not match what's in 'known good' file even though that shouldn't cause the regression test to fail, and thus we don't want h5diff to report them as not equal. Often these objects are specific attributes.