Support LESS-style MDRIVER environment variable for default flags

[llimllib]

Currently mdriver supports individual environment variables for options:

• MDRIVER_THEME
• MDRIVER_WIDTH
• MDRIVER_PADDING

It would be nice to also support a single MDRIVER environment variable that works like LESS, where you can specify command-line flags as a string:

export MDRIVER="--padding 2 --width 100 --theme monokai"

This would be similar to how less allows:

export LESS="-SRXFi"

The individual env vars should be kept for backward compatibility. The precedence would be:

1. Command-line flags (highest priority)
2. Individual MDRIVER_* env vars
3. MDRIVER env var (parsed as flags)
4. Built-in defaults (lowest priority)

This gives users flexibility to choose their preferred configuration style.