[FEATURE] Rename config file to stego.toml

[jgabaut]

Renaming the config file to stego.toml would be pretty easy, and backwards compatibility may be ensured.

We would then have the corresponding stego.lock be a file edited by the program itself.

Relevant amboso issue:

• issue

This is not to be considered for:

• [FEATURE] Feature freeze on amboso 2.1 #172

, since it will come as part of the 2.1 API