Feature: add default_time option

[Guts]

It allows to define a default time which is applied to dates without time.

Benefits

• not necessary to specify a time in the meta section of every single page
• avoid the midnigh publishing pick

Example

Configuration:

[...]
  - rss:
      date_from_meta:
        as_creation: "date"
        as_update: false
        datetime_format: "%Y-%m-%d %H:%M"
        default_timezone: "Europe/Paris"
        default_time: "22:00"
[...]

Page:

---
title: The MkDocs RSS Plugin
authors: ["[email protected] (Julien Moura)", "[email protected] (Tim Vink)"]
date: 2020-07-06
description: "MkDocs RSS plugin: generate RSS feeds for your static website using git log."
image: "rss_icon.svg"
---

Output:

[...]
<pubDate>Mon, 06 Jul 2020 22:00:00 +0200</pubDate>
[...]

[codecov]

Codecov Report

Merging #145 (cfadda8) into main (b7694de) will increase coverage by 0.06%.
The diff coverage is 73.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   81.18%   81.25%   +0.06%     
==========================================
  Files           7        7              
  Lines         372      384      +12     
  Branches       82       84       +2     
==========================================
+ Hits          302      312      +10     
- Misses         44       46       +2     
  Partials       26       26              

Impacted Files	Coverage Δ
mkdocs_rss_plugin/util.py	72.16% <66.66%> (+0.43%)	⬆️
mkdocs_rss_plugin/plugin.py	94.69% <77.77%> (-1.47%)	⬇️

[Guts]

Related discussion #142 (comment)