Implemented negative filters for title and description.

[dop251]

I'd like to filter out episodes with certain keywords in the title or description. Because Go's regexp engine does not support negative lookaheads it is not possible to do this just by specifying a regexp for title.

I considered the following approaches:

• Switching to a different regexp engine (extra dependency, not good).
• Introducing a special syntax for negative filters, e.g. prepending the filter regexp with '!' (hacky).
• Adding separate negative filters.

In the end I've decided in favour of the last approach. This PR adds negative filters for title as well as positive and negative filters for description.

[mxpv]

Thanks!