Add support for nested field indexes

[lggomez]

Following the examples in the integration tests, I'm talking about nested field accessors of the following form:

"sibling_first": &IndexSchema{
    Name:    "sibling_first",
    Unique:  false,
    Indexer: &NestedStringFieldIndex{Field: "Sibling.First"},
},

Rather than including this behavior as the default in the StringFieldIndex (becasue of the performance penalty of object traversal), I added a separate index type, NestedStringFieldIndex

[hashicorp-cla]

All committers have signed the CLA.

[hbagdi]

Oh nice, this is similiar to https://github.com/hbagdi/deck/blob/master/state/indexers/subFieldIndexer.go

[ekovacs]

will this get merged?