Add support for marking files as "generated" by regexp against path

Summary:
Not all auto-generated files can include the magical
"generated" annotation for one reason or another, but they may follow
path rules. This patch allows files to be marked as automatically
generated by matching the path with a regular expression.

Test Plan:
Alter 'differential.generated-paths' setting in config.
Create a new diff that affects a file matching one of those regular
expressions. Verify that Differential marks it as automatically
generated and therefore probably not worth reviewing (in the same way as
the magical "generated" annotation.

Reviewers: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D1455