Add Xresources-based theme

[SqrtMinusOne]

I use Xresources to configure themes. This PR adds themes called xresources-light and xresources-dark, provided that xrdb is found and xrdb -query returns the necessary colors.

This is a bit suboptimal for light theme (which I use), because light themes don't need as much shading as C-o and C-p currently do. But solarized-light also has this issue.

[darrenburns]

I'd want to put this behind a config option use_xresources which is False by default. If that option is True, then this method would be called.

It should also be possible to set the theme to xresources-dark or xresources-light directly from the config file (e.g. theme: xresource-dark).

[darrenburns]

I'd like this to sit behind a config option.

[darrenburns]

Thanks for the contribution! - I've done a short review, let me know what you think or if you need help.

[SqrtMinusOne]

Done. I've:

• add an option called use_xresources, as you suggested;
• changed the method to fail with more verbose errors in case something goes wrong;
• moved the invocation of the method from on_mount to the constructor. I'm not sure if there's a more suitable place for it, but it works.

[SqrtMinusOne]

Oops, my Emacs has auto-formatted the table in the README. I hope you don't mind.

[darrenburns]

Thanks! I'll try to review this this week.

[darrenburns]

Thank you - I've made a few minor changes. Moved stuff into a new module to keep it separate from the App, an simplified the code a bit.

Would you mind giving it a quick test? If it looks good to you I will merge.

[SqrtMinusOne]

Thanks.

But the current version fails with:

  File "/home/pavel/00-Scratch/posting/src/posting/xresources.py", line 36, in load_xresources_themes
    missing_colors = set(XRDB_MAPPING.values()) - supplied_colors.keys()
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: unhashable type: 'list'

The point of list(itertools.chain(*self.XRDB_MAPPING.values())) was to flatten that list. I'll make a new PR with a fix.