Enable alternative themes for Bootstrap (e.g. Boostwatch)

Alchemy is a great theme, but it was not using the full potential of
underlying technology. Bootstrap provides an easy way to customize look
and feel of rendered pages, and a lot of themes for Bootstrap are freely
available, e.g. at <https://boostwatch.com>

This commit introduces a new configuration variable BOOTSTRAP_CSS to make
use of any existing Bootstrap skins. All incompatibilities in `theme.css`
were fixed:

- Instead of using constant color values we use Bootstrap css variables for
  colors:
  <https://getbootstrap.com/docs/4.3/getting-started/theming/#css-variables>
- The only hardcoded color value left is the color for header/footer
  borders. It was moved into a variable `--alchemy-border` for easier
  overriding if needed. I have tested the current value against both dark
  and light Boostwatch themes, it works ok.
- Pagination template was slightly changed to avoid producing invisible
  text with some of Bootstrap skins

README.md

@@ -56,6 +56,7 @@ Visit the [Settings wiki](https://github.com/nairobilug/pelican-alchemy/wiki/Set
 - **DESCRIPTION**: Index HTML head `<meta>` description.
 - **LINKS**: A list of tuples (Title, URL) for menu links.
 - **ICONS**: A list of tuples (Icon, URL) for icon links.
+- **BOOTSTRAP_CSS**: URL of Bootstrap CSS file. Use this to enable Boostwatch themes.
 - **PYGMENTS_STYLE**: Built-in Pygments style for syntax highlighting.
 - **HIDE_AUTHORS**: Hide the author(s) of an article - useful for single author sites.
 - **RFG_FAVICONS**: Use a Favicon Generator package.