(maint) Disable future parser warnings based on :disable_warnings

Work on PUP-2650 missed that the future parser has a separate method of
handling deprecation warnings. This changes IssueReporter to muzzle
deprecations when 'deprecations' is in Puppet[:disable_warnings]. It
also adjusts the max_warnings/deprecations counts to be one-based, so a
setting of Puppet[:max_warnings] = 4 emits at most 4 warnings per
manifest from future parser. A setting of 0 blocks the warnings
altogether.

Adds specs for the current behavior.