Stricter URL checks

[richardlau]

Refs: nodejs/node#26691 (comment)

e.g. for nodejs/node@bdea725

-bash-4.2$ node ../core-validate-commit/bin/cmd.js bdea725 ✖ bdea725bdcb299579547f66ebcc98af16f53cd16 ✖ 4:7 Fixes must be a GitHub URL. fixes-url ✔ 5:7 Valid fixes URL. fixes-url ✔ 0:0 blank line after title line-after-title ✔ 0:0 line-lengths are valid line-length ✔ 0:0 metadata is at end of message metadata-end ✔ 3:8 PR-URL is valid. pr-url ✔ 0:0 reviewers are valid reviewers ✔ 0:0 valid subsystems subsystem ✔ 0:0 Title is formatted correctly. title-format ⚠ 0:50 Title should be <= 50 columns. title-length-bash-4.2$

[richardlau]

Guess I was a bit too strict 😆 : #55