feat: parse default value with tag

[wrfly]

close #68

[coveralls]

Coverage decreased (-0.6%) to 93.29% when pulling 918c260 on wrfly:master into 57836b8 on alexflint:master.

[alexflint]

Thanks for the PR @wrfly

But couldn't this be done by the user of this library? As in

var args struct {
	Foo string `default:"xyz"`
	Bar bool `default:"123"`
}
ecp.Default(&args)
arg.MustParse(&args)

I realize that this is an extra line (or three if you handle errors) on each usage of the library, but I think it's preferable to do it that way because it doesn't commit us to a particular defaults library, and users can therefore choose a different defaults library if they wish to.

It also leaves open the approach I currently recommended:

var args struct {
	Foo string
	Bar bool
	Baz MyStruct
}
args.Foo = "xyz"
args.Bar = true
args.Baz = MyStruct{ ... }
ecp.Default(&args)
arg.MustParse(&args)

This approach is a little less elegant, but it does work for user-defined structs such as MyStruct above.

[wrfly]

@alexflint You are right, at the first beginning, I thought as you do, it's just a extra line, but somehow I thought it's kinda little ugly, so...
But since you said so, I'm OK with this. 😄
I'll close this PR and comment at #68