Is there a Twitter app that allows filtering of tweets based on source, hashtag, etc?
Touiteur has been updated today and now has filtering. Please note that this feature may just be the paid version. I'll update this answer with a link and QR code when I've confirmed those details.

Edit: It would seem that filtering isn't a premium feature.

The filtering can be found under the mute settings in Touiteur -> More -> Mute.
You can choose to mute text (inc. hashtags), users and apps (sources).

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