Append to existing "filters" field.
Before this change we would have replaced an existing "filters" field with the filters applied in this run. But if the user runs `onionperf filter` more than once to apply different filters, we'll want to list all filters in the "filters" field. This change also removes a couple of `self.`s, in particular `self.analysis` and `self.filters`, because one Filtering object can apply filters to more than one analysis.
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