Add possibility to define trimming rules and then trim superfluous parts of markdown produced based on them, automatically (based on URL) or as a button

[prscns]

If a user repeatedly saves pages from a certain website / that fit a certain format, it will often produce a lot of superfluous text. Trimming it automatically (based on URL) or at the press of a button would be very useful.

The trimming could involve user-defined rules, such as for trimming everything before stringA, after stringB, or between stringC and stingD.

Websites where this would be useful include social media, like twitter. Many different services try to scrape tweets, but they're often pricey, unreliable, or have privacy issues. MarkDownload would have many advantages over them if it allowed trimming superfluous text this way.