Implement formatters for log

[bartlomieju]

Based on discussion in #39 I'm making first pass at implementing formatters for log module.

This is already working and here are examples (they have exactly same output)
1.

new FileHandler("INFO", { 
    filename: "log.txt", 
    // simple string-based formatter, interpolates with fields of LogRecord
    formatter: "{levelName} {datetime} {msg}" 
});

new FileHandler("INFO", { 
    filename: "log.txt", 
    // function-based formatter, you can do
    // whatever you want here
    formatter: logRecord => {
         return `${logRecord.levelName} {logRecord.datetime} {logRecord.msg}`;
    }
});

CC @ry @hayd

[bartlomieju]

this is just placeholder filename, I want to know your opinion

[ry]

Not so happy with having an options interface with only one item in it. (cc @piscisaureus)
It seems if you're constructing a FileHandler, specifying a filename should be part of the process - i.e. no default value.

[bartlomieju]

Actually it's two items, because FileHandlerOptions has filename and formatter. +1 for no default value

[ry]

LGTM