move helpers.rchop to utils.rchop
add lchop function that works like rchop but for prefixes
summarize/intro filter ignore img, audio & video tags per default, closes #124
ignored -> ignore
extend intro and summarize documentation, move conf.py values to docs/filters.rst
move docstrings to the actual Reader object
include filter configuration namespace into hash ...

... and allow the inclusion of certain environment/configuration
variables into the hash, too. For example could you include the
modification timestamp of an external file, so Acrylamid automatically
updates the content (for example within the acronym filter).

The environment has now a new attribute `.filter` that contains the
actual used filters. This way, the environment hash depends on the
filter hash as well and can trigger re-compilation under given
circumstances.
include used environment attributes into filter hash, related to #118
update CHANGES.md
Merge branch 'feature/filter-improvements', closes #123