Add filtering by regex matching in ImageFileCollection

[mwcraig]

This closes a couple of issues by allowing regular expression patterns as the values in files_filtered and headers, hdus, etc.

For example, #480 asked for the ability to select objects that are either "flat" or "dome". This could now be done with:

image_coll = ImageFileCollection('rawdir')
fl = image_coll.hdus(return_fname=True, regex_match=True, object='FLAT|dome’)

The regex searches are case insensitive to match the behavior of the existing behavior.

This addresses part of #595 (the wildcards in the values part).