Removal of custom log and use env_logger (or similar) instead

[aldoborrero]

Rationale

In treefmt, there's a custom instantiation of a CustomLog like the below code snippet:

pub struct CustomLog {}

impl log::Log for CustomLog {
    fn enabled(&self, _metadata: &Metadata) -> bool {
        // The log crate already has log::set_max_level to filter out the logs.
        // We don't need more than that.
        true
    }

    fn log(&self, record: &Record) {
        match record.level() {
            Level::Trace => eprintln!("{}: {}", style("[DEBUG]").bold().dim(), record.args()),
            Level::Debug => eprintln!("{}: {}", style("[DEBUG]").bold().dim(), record.args()),
            Level::Info => eprintln!("{}: {}", style("[INFO]").bold().dim(), record.args()),
            Level::Warn => eprintln!("{}: {}", style("[WARN]").bold().dim(), record.args()),
            Level::Error => eprintln!("{}: {}", style("[ERR]").bold().dim(), record.args()),
        }
    }
    // ignore, stderr is already flushed by default
    fn flush(&self) {}
}

The library crate should be logger agnostic, delegating the task of instantiating the logger implementation to the binary crate or, if treefmt is used as a library, to the implementor.

We can achieve the same functionality by instantiating env_logger in the main binary crate, which allows easy customization with its Builder class.

Related features we can tackle whilst implementing this one are also related to bringing colored logs.

[brianmcgee]

Makes sense to me. I don't mind having a look at it, since #191 is a bit blocked at the moment.