Set action of --filter to 'append'
Set destination of --filter to 'filters'
Update help text of --filter
Discard images not taken in any of the given --filters
Show the percentage of selected and discarded images
Allow to use the --filter option multiple times

Replace the 'store' action with 'append', so that the --filter option
can be used multiple times: every time that optparse encounters the
option, the photometric filter is appended to the destination. This
makes it possible to specify not only one, but multiple filters on
which photometry will be done — while the rest are ignored.

This merges branch 'multiple-filter-options'