Control how numbers on their own are treated

Hi there,

This pull request includes one bug fix and one 'feature':
1. If you parse the string "1st" (just an ordinal) then it was previously ignoring the "now" option (which is kind of the whole reason I would want to use Chronic) - I've now fixed this

2. If you parse the string "1" (just an integer) then at present it always treats that as a time (1pm) - but my use case doesn't care about times, only ever dates. I've added an option "ambiguous_number_priority" which you can set to either :time (the default, current behaviour) or :date (which would treat "1" identically to the ordinal "1st")