Pull all Shiftr specific parsing out of the PathElement classes so that they can be reused by Defaultr

[milosimpson]

[milosimpson]

Would be interesting to see if we would use an ANTLR parser.

[milosimpson]

This was done and powers the https://github.com/bazaarvoice/jolt/releases/tag/jolt-0.0.22 Modify transform.