Supersedes #104, closes #97

Most of the stuff, discussed in #104 applied here. We still first take CLI arguments, then config values and only then some defaults to make decisions. Again, by default config.toml does not exist and it's OK to have all options commented out/unset.
Default behavior is mostly unchanged, minor non-breaking behavior changes listed below:

• change: cli.log_level now is a Option<Level> with fallback value Level::Info in config. No changes from user perspective whatsoever
• feat: add cli.config parameter. Defaults for fallback are also added
• change: in case cursor and clipboard are false (default behavior) -> read values in config.toml (defaults for these values are false as well, so current users won't notice the difference)
• change: stdout, clipboard and FILE doesn't conflict with each other anymore. Screenshot file will not appear on disk now only if no FILE provided via args and file in config.toml is false at the same time. In all other cases file is saved, with path following this priority: FILE > config.file.path > current directory. Not breaking, but two category of users affected. First, users currently using - as FILE will notice that aside from stdout, there's a screenshot file in current directory (if no config edits were made). Second catogory is users, that did not provide FILE but provided --clipboard argument: these users will see similar behavior as above