Feature request: add support for "gtfo"

[kereyroper]

An owner may notice that a contributor has opened a PR that will result in immediate issues if merged. This is common when unit tests provide an insufficient level of validation, or if there are logical conflicts between the version of the theoretical merge against which Travis or another CI system has run unit tests, and the version which will result from multiple merges being actually performed that do not manifest as code-level merge conflicts.

github-lgtm should support adding a comment, perhaps "gtfo," which would disallow merging of the PR until the issue has been corrected.

[octobeard]

I could see this being very useful. Especially appreciate the suggested comment syntax.

[chase-seibert]

Good suggestion.

[jemiahlee]

Alternate syntax suggestion: 🙅

[octobeard]