Doc how to convert from logutils

[josephholsten]

There are two parts required to migrate from logutils:

• Initial setup: defining minimum log level to filter, &c. Showing the equivalent of the example from logutils README should be sufficient.
• Converting message format: all logging calls from [Level] to Level:. Is there a way to to use the messages in logutils format?

[josephholsten]

Lol. Entering tickets from my phone is the worst.

[d2g]

By setting the logger to take the square format it should just work.
logfilter.SetParsers([]logfilter.Parser{logfilter.SqrParser})

Agreed needs to be better documented.

[josephholsten]

how does this look for an updated usage section:

package main

import (
	"fmt"
	"log"

	"github.com/d2g/logfilter"

	"github.com/d2g/logfilter/dummy"
)

func main() {
	// Allow messages at warning and above
	logfilter.Default(logfilter.Warning)
	// Include logs from a package, set a lower level
	logfilter.Include("github.com/d2g/logfilter/dummy").When(logfilter.Info)
	// Exclude logs from a particularly noisy package
	logfilter.Exclude("github.com/d2g/logfilter/noisy")
	// Use "[WARN] " style prefixes, like logutils
	// logfilter.SetParsers([]logfilter.Parser{logfilter.SqrParser})

	// Unformatted log messages
	log.Print("Message I haven't updated") // this will not print
	// With logfilter prefix
	log.Print("Trace: stdlib Tracing")   // this will not print
	log.Print("Debug: stdlib Debugging") // neither will this
	log.Print("Info: stdlib Informing")  // nor this
	log.Print("Warning: stdlib Warning") // this will print
	log.Print("Error: stdlib Erring")    // so will this
	log.Print("Fatal: stdlib Fatality")  // and this

	// Logging from a package
	dummy.Unformatted() // this will not print
	dummy.Trace()       // neither will this
	dummy.Debug()       // nor this
	dummy.Info()        // this will print
	dummy.Warning()     // so will this
	dummy.Error()       // and this
	dummy.Fatal()       // and this
}

Also, any ideas on how to get godoc2md to use a code block like:

```go
...
```

[d2g]

Let me know if you think that's enough.