fix(scripts): Generate semver tag within post-release.sh (#263)

This commit modifies post-release.sh to auto-generate a semver tag based
on the highest tagged version of material-components-web by lerna. E.g.,
'material-components-web@0.4.0' would be translated into a `v0.4.0` repo
tag.

This mainly addresses the issue where standard-changelog fails to
generate a correct changelog due to lack of semver tags. However, having
an auto-generated semver tag for the _entire_ repo seems useful both for
automated tools relying on semver, and also for non-automated humans
looking to get a general worldview of our latest release :)

Fixes #209
[ci skip]

Author: traviskaufman

scripts/post-release.sh

@@ -28,4 +28,11 @@ git add CHANGELOG.md
 git commit -m "docs: Update CHANGELOG.md"
 echo ""
 
-log "Done! You should now git push to master"
+# Extract repo version from material-components-web@Maj.min.patch tag generated by lerna
+REPO_VERSION=$($(npm bin)/semver $(git tag -l | grep material-components-web | sed 's/material-components-web@//') | tail -n 1)
+SEMVER_TAG="v$REPO_VERSION"
+log "Tagging repo using semver tag $SEMVER_TAG"
+git tag $SEMVER_TAG -m "Material Components for the web release $SEMVER_TAG"
+echo ""
+
+log "Done! You should now git push to master and git push --tags"