ENH: Hack-y update to allow easier user plotting

- all `scan_args` passed to the run engine are now put in the
  `custom` field of the run header.
- Replay will look for `plotx` and `ploty` keys in the run header
  and automatically plot those data accordingly if those keys
  exist. These are changes requested by @stuwilkins
- Minor update to from __future__ imports

MNT: Fixed crappy attempt at setting x/y

MNT: Removed scanx as an attribute

- Also reverted changes to scan_engine re: Daron

MNT: Reverted to dumping **scan_args