Add tests for utils / setting up INI file

[sacha-l]

Unit tests that cover some items in #13 for the util functions for reading/writing to an INI file:

• string_to_vec works
• string_to_hashmap works
• write_config works
• read_ini_file works (with custom config and default fallback i.e. without ports or auth specified)

This PR also adds a few fixes to the library that were caught while writing tests, such as:

1. generating a new keypair if none was read from the file
2. fallback to default ports specified in prelude.rs if none were specified in the INI file
3. rust fmt had the wrong extension 🙂‍↕️

Note, related to #7 and #15: we should let users know that BootstrapConfig will default to generate the keypairs in generate_keypair_from_protobuf even though the INI file doesn't have the required [auth] field. We may want to enforce that users have this compulsory field by panicking with a log message or we don't panic but log that the fallback was used. WDYT @thewoodfish ?

Please merge once reviewed =)

[thewoodfish]

Great. I think we should let the [auth] section be optional and then log when it is absent, that we generated a default keypair and in turn a network identity.
The tests all work great.