implement a rudimentary Atom/RSS feed module

this provides a somewhat unconfigurable (at the moment) feed module
which provides Atom and RSS feeds. entries are determined by symlinks to
content pages, because my core CMS usage is still more general and not
blog-like. the symlinks allow for arbitrarily adding entries as I see
fit.

this also moves core Markdown parser stuff to the library module, since
that's used by the feed as well as normal pages

Signed-off-by: Brian S. Stephan <bss@incorporeal.org>

requirements/requirements-dev.txt

@@ -35,6 +35,8 @@ dlint==0.14.1
     # via incorporeal-cms (pyproject.toml)
 dparse==0.6.3
     # via safety
+feedgen==1.0.0
+    # via incorporeal-cms (pyproject.toml)
 filelock==3.13.1
     # via
     #   tox
@@ -83,6 +85,8 @@ itsdangerous==2.1.2
     # via flask
 jinja2==3.1.2
     # via flask
+lxml==5.0.0
+    # via feedgen
 markdown==3.5.1
     # via incorporeal-cms (pyproject.toml)
 markdown-it-py==3.0.0
@@ -144,6 +148,8 @@ pytest==7.4.3
     #   pytest-cov
 pytest-cov==4.1.0
     # via incorporeal-cms (pyproject.toml)
+python-dateutil==2.8.2
+    # via feedgen
 pyyaml==6.0.1
     # via bandit
 requests==2.31.0
@@ -158,6 +164,8 @@ safety==2.3.5
     # via incorporeal-cms (pyproject.toml)
 setuptools-scm==8.0.4
     # via incorporeal-cms (pyproject.toml)
+six==1.16.0
+    # via python-dateutil
 smmap==5.0.1
     # via gitdb
 snowballstemmer==2.2.0

requirements/requirements.txt

@@ -8,17 +8,25 @@ blinker==1.7.0
     # via flask
 click==8.1.7
     # via flask
+feedgen==1.0.0
+    # via incorporeal-cms (pyproject.toml)
 flask==3.0.0
     # via incorporeal-cms (pyproject.toml)
 itsdangerous==2.1.2
     # via flask
 jinja2==3.1.2
     # via flask
+lxml==5.0.0
+    # via feedgen
 markdown==3.5.1
     # via incorporeal-cms (pyproject.toml)
 markupsafe==2.1.3
     # via
     #   jinja2
     #   werkzeug
+python-dateutil==2.8.2
+    # via feedgen
+six==1.16.0
+    # via python-dateutil
 werkzeug==3.0.1
     # via flask