merged branch Seldaek/date_modify (PR #757)

Commits
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ae85586 Add docs for date_modify
0366cf6 Fix docs of date filter

Discussion
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Date modify filter

This is probably possible via `date(foo)|modify('-1day')|format()`, but it's non obvious and a dedicated filter might be a bit more prominent.

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by fabpot at 2012-06-22T07:01:32Z

Just a quick thought: As it modifies the date, it should probably be a function, not a filter.

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by stof at 2012-06-22T07:24:52Z

@fabpot Aren't filters meant to transform the value ?