Add ability to call programmatically

[bhilbert4]

Currently mkrefs.py can only be called from the command line, meaning that the location of mkrefs.py must be added to PATH if you want to be able to use it from anywhere. This somewhat defeats the purpose of installing the package.

Add the ability to call jwst_reffiles programmatically. Something along the lines of:

m = jwst_reffiles.mkrefs()
m.cfgfile = 'my_file.cfg'
m.run()