Add fairly fine grained filtering support

Add fairly fine grained filtering support

Now the program can make only one query but it can accept multiple
optional filters based on the fields. Each extra argument is a filter
specification, and filters are all joined using a logical AND.

The individual filters have the format X:fitler, there X is a field
specification:
t = titulo
d = desc
a = autor
f = formato
c = comentarios
C = cds
F = fecha
D = downloads

And filter is a string that should be found on that field (case
insensitive). If the format specifier is not known (or there isn't one)
the filter string is looked in all the fields.