This updates Doc8Tool to subclass the newer BaseTool, and to use the

newer features it offers (dependencies, improved configurable file

extension handling, centralized base command building, and standardized

error code output).
The new version of the tool has safer report parsing, using a regex

rather than manual string splitting and parsing to match lines. This

simplifies things quite a bit and avoids possible errors, or crashes if

doc8 changes output someday.
Unit tests were added to cover the new doc8 parsing, and to provide

integration tests.

Unit tests pass on Python 2.7 and 3.x.