Implement theme command.

[dylanfprice]

Fixes #1588. Took inspiration from #1603.

Testing

I switched back and forth between a few different themes I have and visually it seemed to work great. I also was able to switch back to the default theme with :theme default.theme.

I switched to the same theme I was already using and verified that it didn't rebuild the buffer (e.g. the thread I was viewing didn't reset its collapsed state).

I tried loading :theme doesntexist and saw the error message in the UI.

I sabotaged one of my theme files and tried to load it. I saw an error message in the UI and this in the logs:

ERROR:theme:Theme file `/home/dylan/.config/alot/themes/solarized-light` failed validation: key "footer" in section "global" is missing.

I added a few unittests for the get_theme function I wrote.

Docs

I took a stab at updating the docs, but didn't try to build it or anything. I didn't realize the command docs were generated, fixed.

[dylanfprice]

@pazz the workflow should pass now

[dylanfprice]

@pazz @lucc

[lucc]

This seems to work for me and to code looks good.

@pazz you were in favor of this in #1588, so I will leave the rest to you.

[pazz]

thanks