Override config options from the command line

[inducer]

It'd be nice if config values could be overridden from the command line. My use case is that I'd like to change my theme based on time of day/whether dark mode is enabled in my DE. Some other approaches that might help me achieve the same thing:

• Execute commands from the command line (i.e. :set for me)
• Allow environment variable expansion in the configuration

Either of the two feels like a bigger can of worms than the config overrides.

[kirawi]

See #2158

Helix will reload the config if it receives the USR1 signal.