Add --gitlab option to mh_style and mh_lint

[LucasDooms]

This adds a new output format for the mh_style and mh_lint commands.
See https://docs.gitlab.com/ci/testing/code_quality/#code-quality-report-format for the specification.

The new code is adapted from the existing JSON handler with different to_gitlab() calls, and self.blob is a json list instead of an object.

I tested it on gitlab and it seems to work.

[pvogt09]

I didn't check whether the code actually runs, but it looks like a valuable additional feature and it would be nice to have it integrated in a future version.

[pvogt09]

Suggested change

	"check_name" : check_name,
	"type": "issue",
	"check_name" : check_name,
	"categories": ["Style"],

Eventually map categories in https://github.com/codeclimate/platform/blob/master/spec/analyzers/SPEC.md#data-types to Message.kind?

[pvogt09]

Suggested change

	rv["lines"] = {"begin": self.line if self.line else 0}
	if self.col_start and self.col_end:
	rv["positions"] = {
	"begin": {
	"line": self.line if self.line else 0,
	"column": self.col_start if self.col_start else 0,
	},
	"end": {
	"line": self.line if self.line else 0,
	"column": self.col_end if self.col_end else 0,
	},
	}
	else:
	rv["lines"] = {"begin": self.line if self.line else 0}