Allow default values in struct tags

[alexflint]

This pr introduces support for default values specified as struct tags:

var args struct {
  Foo int `default:"123"`
}
arg.MustParse(&args)

This means there are now two ways to provide default values. The old way still works as expected:

var args struct {
  Foo int
}
args.Foo = 123
arg.MustParse(&args)

The reason I'm introducing this feature is that it has now been requested many times, and the old approach does not work with subcommands (introduced in #82; see discussion in #86).

Fixes #40 #68 #86 #92

[atc0005]

This pr introduces support for default values specified as struct tags:

var args struct {
  Foo int `default:"123"`
}
arg.MustParse(&args)

Does this result in the default values being shown in the help text? I tested using commit 5d3ebcc, but the default "labels" only show when setting them via the "old way".