osmpbf: add filter functions

[paulmach]

This is the completion of #27

Adds filter functions

type Scanner struct {
	// If the Filter function is false, the element well be skipped
	// at the decoding level. The functions should be fast, they block the
	// decoder, there are `procs` number of concurrent decoders.
	// Elements can be stored if the function returns true. Memory is
	// reused if the filter returns false.
	FilterNode     func(*osm.Node) bool
	FilterWay      func(*osm.Way) bool
	FilterRelation func(*osm.Relation) bool

	// contains filtered or unexported fields
}

@oflebbe