Remove unnecessary .rb from require.
Add config option to raise errors on deprecations.
Add notice about the new option so users are made aware of it.
Add changelog entry for new config option.
This is more of an enhancement than a bug fix.
Rename deprecation printer classes.

For the new `raise_errors` option, we were using
the FilePrinter but not actually giving it a File
object. We use it because it immediately prints
deprecations, whereas the IOPrinter delays the printing
of errors until the end, so I think these names are
better.
Merge pull request #1161 from rspec/turn-deprecations-into-errors-master

Turn deprecations into errors