feat: Include theme colors in theme object

[gadenbuie]

Also refactors shinyswatch.theme.___ into a ShinyswatchTheme object.

1. Read key theme colors from the bslib theme object
2. Those are saved as the bsw5_theme_colors dict in _bsw5.py.
3. Themes are now a callable ShinyswatchTheme class that include a .colors property.
4. (Incidentally themes are also now tagifiable.)
5. The colors property is actually a ThemeColors class that makes the theme's colors available as properties on the class, e.g. shinyswatch.theme.cosmo.primary.

>>> from shinyswatch.theme import cosmo
>>> cosmo.colors
<ThemeColors('cosmo'): 
 body_color : #373a3c
 body_bg    : #fff
 light      : #f8f9fa
 dark       : #373a3c
 primary    : #2780e3
 secondary  : #373a3c
 info       : #9954bb
 success    : #3fb618
 warning    : #ff7518
 danger     : #ff0039
>
>>> cosmo.colors.info
'#9954bb'
>>> cosmo.colors.success
'#3fb618'

I also updated the documentation in the index to use shinyswatch.theme.minty instead of shinyswatch.theme.minty() and I added an example where the theme's colors are used to theme a plot.

Note: I did not update the example app included in each theme's docs.