Implement encoding.TextUnmarshaler in Path

Hi, it would have been nice to have `UnmarshalText([]byte) error` on `Path`.
That would allow for easy construction i.e from yaml document:

```yaml
myQueries:
  - $.store.book[*].title
```

```go
type myData struct {
  MyQueries []*jsonpath.Path  `yaml:"myQueries"`
}
```

Caller just needs to have YAML document un-marshaled.