Add method for setting default file or writer

[RadhiFadlillah]

The main focus of this PR is to add new method for setting default output file if there are no output defined for some or all log level. This method is especially useful if you want to put all log level to single file.

package main

import (
	"github.com/RadhiFadlillah/lfshook"
	"github.com/sirupsen/logrus"
)

func main() {
	// Create lfs hook
	hook := lfshook.NewHook(nil, nil)
	hook.SetDefaultPath("general.log")

	// Connect hook with logrus
	logrus.StandardLogger().AddHook(hook)

	// Create some log
	logrus.Infoln("This is info")
	logrus.Warnln("This is warning")
	logrus.Errorln("This is error")
	logrus.Fatalln("This is fatal")
}

Code above will make a general.log file which contains all logs :

time="2017-12-10T22:11:22+07:00" level=info msg="This is info"
time="2017-12-10T22:11:22+07:00" level=warning msg="This is warning"
time="2017-12-10T22:11:22+07:00" level=error msg="This is error"
time="2017-12-10T22:11:22+07:00" level=fatal msg="This is fatal"

While adding this method, I fixed some bugs :

1. lfshook will fail to create new log file if target directory doesn't exist.
2. The formatter that pased through constructor won't disable color mode in TextFormatter, which make log files looks weird.

[rifflock]

This definitely makes it cleaner than having to create a map of identical paths/writers.

Ideally we'd be able to set a default path AND a path map (or writer and writer map) in the constructor, but I'd hate to change the constructor signature again.

[rifflock]

If we go this path it might be good to instead support a case for string (default path) and a case for io.Writer (default writer).

I'm not a big fan of allowing the user to successful create a hook that has no valid output paths configured.

[RadhiFadlillah]

Done on 83da246.

[rifflock]

I tried to resolve the merge conflicts to the Readme, but it wouldn't let me (probably because it's your branch). If you can fix those then I can merge it.

[RadhiFadlillah]

@rifflock done.

[rifflock]

This looks good. Should be a non-breaking change.