This updates ShellCheckTool to subclass the newer BaseTool, and to

use the newer features it offers (dependencies, improved file pattern

support, central command building, new loggers, and new comment

options).
The list of file types and shebangs supported have been extended, based

on shellcheck's advertised capabilities.
The handling of shellcheck results has been improved. We now request

JSON output, and use that to convey column, error code, and severity

information.
The payload also provides suggested fixes, in the form of "replacement"

ranges. These are used to modify the referenced line(s) of code and

patch them up to provide a possible replacement line of code. The new

tool support will provide the resulting replacements in the comment, if

we're able to successfully apply them without any issues. This is

future-proofed to back out of the attempt if we see anything we don't

expect from shellcheck.
If anything goes wrong with parsing files or handling settings, the

resulting error is left as a comment on the file.

Unit tests passed on Python 2.7 and 3.x.