User configuration

[sharkdp]

Things that a user might want to configure:

• REPL Prompt
• whether or not to show the "Numbat" font
• Whether or not to pre-fetch currency exchange rates
• Pretty printing: on/off
• Paths to the module folder
• Localization settings (decimal separator)
• whether or not to use unicode exponents like ², ³, … instead of ^2, ^3
• number of significant digits that are displayed

See also: #55

[sharkdp]

https://docs.rs/toml/latest/toml/#deserialization-and-serialization

[sharkdp]

Current plan:

Location of the config file:

• We already have get_config_path() in numbat-cli/src/main.rs. See https://numbat.dev/doc/cli-customization.html#config-path for details where this is located on different systems.
• We'll assume that a config file config.toml is located in that folder. If it is absent, we do not create it — we just use default settings. We never write to that file.

Format: TOML, unless there are very good reasons for another choice

Implementation:

• Create a Config struct in numbat-cli/src/config.rs. Use serde and the toml crate to automatically derive deserialization functions for the config struct.