date: Check all link lines

(rather than just the first one with a date)
date parsing: Better accept standard subscribable pages spec
* fix parsing line lines with tabs
tests: try to debug kel's gemlog date issue
docs: turn into gemini directory
Readme: Rename to markdown extension
License: License under AGPLv3
Readme: Add file to explain design differences from spacewalk
gemini request: wrap more in try block
link parsing: improve url parsing
getdesc: improve tolerance of link line parsing
description parsing: split at colon
Url checking: only check gemini urls
template: be able to replace existing dates
retrieval: tolerate non-bytes data
zachwalk: use input as a template
zachwalk: also read from stdin
dependencies: add requirements.txt file for dateutir dependency
connection: timeout after 2 seconds