Add optional i18n.toml configuration file for the CLI tool

[jannistpl]

Currently, the CLI tool i18n only supports reading the configuration from the Cargo.toml file in the root source directory.

However, in larger projects, the Cargo.toml file can quickly become very full and confusing with options from various tools.

With this PR, it is now possible to optionally create an i18n.toml file in the source root, from which the configuration can be loaded instead.

For example:

# i18n.toml
[i18n]
available-locales = ["en", "de"]
default-locale = "en"
load-path = "locales"

The priorities are then as follows:

• If i18n.toml exists in the source root, load the configuration from there.
• If i18n.toml doesn't exist, load the configuration from Cargo.toml.

[huacnlee]

Sorry, we don't want induction new things, we want to keep this crate simple.