Implement work with slices

[awskii]

For now, we can't pass slice of pointers to struct to parse data to, instead of that we should pass one pointer to struct at itreration.

Current state:

var dest []*MyStructure
for i := 0; i < len(dest) && i < len(lines); i++ {
  if err := hunkee.ParseLine(lines[i], dest[i]); err != nil {
    fmt.Println(err)
  }
}

With ability to parse lines into slice becomes next:

var dest []*MyStructure
// dest len will be extended as much as needed to
// parse all presented lines into
if err := hunkee.Parse(lines, dest); err != nil {
  fmt.Println(err)
}