Allow prefix for semver tags

Hi,

Context: JS monorepo. Lerna tags new releases with the package name, e.g. @scope/[email protected]. While $DRONE_TAG is correctly set to the whole tag, $DRONE_SEMVER is empty, as the tag is not just a semver. I consider making a custom Docker image which would add RUN export DRONE_SEMVER=$(echo $DRONE_TAG | sed --regexp-extended 's/^[email protected]//').

But this could benefit a lot of other projects too. For now, the runner behavior (runner-go/semver.go at 0bd0f8fc31c489817572060d17c6e24aaa487470 · drone/runner-go · GitHub) is to remove the prefix v. Would you consider testing if env vars are set (e.g. DRONE_RUNNER_TAG_SEMVER_REGEX and DRONE_RUNNER_TAG_SEMVER_REGEX_REPLACEMENT) and in this case do something like regexp.MustCompile(...).ReplaceAllString(...)?