Move reusable part of the description of --filter to defaults.py
Move InputFITSFiles class to fitsimage.py
Update InputFITSFiles class docstring
Add --filter option
Add optparse.OptionGroup 'key_group'
Add --filterk option
Use InputFITSFiles to map each filter to a list of FITSImage objects
Ignore FITS images not taken in --filter
Add Bash completion for --filter / --filterk
Iterate over the sorted keys of InputFITSFiles
Add --filter option — combine images taken in a specific filter

This merges branch 'add-filter-option-to-mosaic'
Update Passband module docstring — mention user-defined filters
Include '~' in CONFIG_FILENAME (~/.lemonrc)
Add user-defined filters to the description of the --filter option