Add summary if available for each post
This has been sort of mentioned in #6 but since the issue is taking a long time I made a separate one.
Description
The post struct has a description field which is currently not present in the generated HTML.
A config parameter could be added for the preferred max length of a description.
Notes
Some feeds seem to be using a description tag instead of the official summary tag so might as well try and parse both.
Base functionality implemented in ad0f95559c69553e95f6ff8f262f3188bd8c2423. Will close once the note is also implemented.
Note implemented in cd8168e1d83fd70905d50bb8b5f161104fec2fe4. This will not be added yet at HTML inside of the descriptions often breaks the formatting.