add support for struct field tags

[aerth]

With these changes, one can add a struct field tag pretty:"-" to ignore pretty printing a particular field.

For example, the "z" field would not be pretty printed in the following struct

type testFieldTag struct {
	x, y int
	z string `pretty:"-"`
}

Now print long embedded structs prettier

[aerth]

rebased and refactored... added a few test cases. seems to be working good

[NicklasWallgren]

Can we get this merged?

[sean-]

Bumping this with a +1. When debugging a library that embeds a very complicated, external data structure, the volume of output from external dependencies can dwarf the local code, making pretty's output more difficult to digest than it needs to be.