A common pattern in Review Bot tools is to allow users to set a list of

supported file extensions. The tools that support this all do it a bit

differently, in terms of what they expect or allow, and how they set it.
This change introduces a new mixin, FilePatternsFromSettingMixin,

which standardizes how this works. It offers three new class attributes

for a tool:


file_extensions_setting - The name of a configurable tool setting

  where custom file extensions are set.


file_patterns_setting - The name of a configurable tool setting

  where custom glob file patterns are set.


include_default_file_patterns - Whether any class-level defaults for

file_patterns should always be included in the final result.


This simplifies tools, leading to fewer differences in behavior, and

encourages more customization.
The new implementation handles escaping of file extensions in order to

avoid issues with patterns, as well as handling inconsistencies in

lists of extensions and patterns (extra commas, whitespace, or duplicates).

Unit tests pass on Python 2.7 and 3.x.