Add support for Gitlab CI

[nejec]

• adding support for Gitlab CI

[sethvargo]

It's more efficient to use a struct instead of a bool, since the empty struct does not allocate

Suggested change

	var globalKeywords = map[string]bool{
	"default": true,
	"include": true,
	"stages": true,
	"variables": true,
	"workflow": true,
	}
	var globalKeywords = map[string]struct{}{
	"default": {},
	"include": {},
	"stages": {},
	"variables": {},
	"workflow": {},
	}

[nejec]

Thanks for comments. I have updated the PR and added example testdata/gitlabci.yml.

[sethvargo]

Thanks!