Agate type inference is too aggressive

[drewbanin]

Via: http://agate.readthedocs.io/en/1.6.0/api/data_types.html

All:

Actual:
  null_values = ('', 'na', 'n/a', 'none', 'null', '.')
Desired:
  null_values = ('null')

Boolean:

Actual:
  true_values = ('yes', 'y', 'true', 't', '1')
  false_values = ('no', 'n', 'false', 'f', '0')
  null_values = ('', 'na', 'n/a', 'none', 'null', '.')
Desired:
  true_values = ('true')
  false_values = ('false')
  null_values = ('null')

TimeDelta

Actual:
  null_values = ('', 'na', 'n/a', 'none', 'null', '.')
Desired:
  null_values = ('null')

This would be a breaking change, so I think it's something we ought to do sooner rather than later.

[cmcarthur]

document the heck out of it

[beckjake]

fixed in #905