Add optional TOML config file support via --config flag

[aufam]

Hello, thanks for making this awesome project.

I frequently find myself running agg with the same set of flags over and over. So I figured a config file would be a nice quality-of-life improvement.

This PR adds support for an optional TOML configuration file, making it easy to persist preferred settings in one place.

Usage

# Explicit config path
agg --config path/to/agg.toml input.cast output.gif

# Auto-detected from default config directory (no flag needed)
agg input.cast output.gif

# CLI flags still override config values
agg --speed 2.0 input.cast output.gif

If --config is not specified, agg will look for agg.toml in the user's config directory (via the dirs crate):

If no config file is found, everything works exactly as before — no breaking changes.

Priority order

CLI arguments > config file > default values

Explicitly passed CLI flags always win. The --help output continues to display default values as before.

Config file format

The config struct mirrors agg::Config and uses kebab-case keys:

font-family = "JetBrains Mono"
font-size = 14
line-height = 1.4
speed = 1.0
no-loop = false
idle-time-limit = 5.0
fps-cap = 30
last-frame-duration = 1.0

Changes

• Added --config CLI flag
• Added TOML config file parsing via serde and toml crates
• Added dirs crate for resolving the default config directory

Happy to adjust anything — thanks for your time reviewing!