Feed Summary

This plugin allows article summaries to be used in RSS and Atom feeds instead of the entire article. The plugin modifies the `Writer` class and then sets `Pelican.get_writer()` to this modified class after initiation. The plugin is enabled by setting `'FEED_USE_SUMMARY'` to `True` in `pelicanconf.py`.

The length of the article can be set via three methods:
1. The tags used for the `summary` plugin, i.e. `<!-- PELICAN_END_SUMMARY -->`.
2. A setting called `'FEED_SUMMARY_LENGTH'`, which determines the number of words from the beginning of the post.
3. The value of `'SUMMARY_MAX_LENGTH'`, which works in the same way as `'FEED_SUMMARY_LENGTH'`.
4. If none of these criteria are met, the length of the summary defaults to 50 words.

This plugin is particularly useful for those, like myself, who include IPython notebooks as part of blog posts. These notebooks store images inline by default, which can result in extremely large file sizes for feeds.