Improve compare tool with more UI improvements to help with

generating GIFs for reviewing PRs.

Add query params to the compare tool. For example, open the following in
a local browser:

vexflow/tools/compare/index.html?currentLabel=PR-1480&referenceLabel=4.1.0

Author: ronyeh

tools/compare/README.md

@@ -0,0 +1,47 @@
+# Compare Images
+
+First, open index.html in your local browser.
+
+When we refer to "current" images, we mean the test output from your current branch.
+
+We want to compare the current images to "reference" images from an earlier revision (usually the most recent npm release).
+
+There are two ways to generate images to compare.
+
+# Method 1: Single Working Copy
+
+First, check out a known good revision. Then build that revision and move the output to the `vexflow/reference/` directory.
+
+```
+grunt reference
+```
+
+Then, check out your current branch and build it.
+
+```
+git checkout my-new-feature-branch
+grunt
+```
+
+Now we need to generate images for both the reference and the current branch.
+
+```
+grunt generate:current
+grunt generate:reference
+```
+
+Open the `vexflow/build/images/` directory and you'll see two folders: `current/` and `reference/`.
+
+You can drag this `images/` folder onto the compare page. It will load the images from the two folders and allow you to compare them.
+
+# Method 2: Two Working Copies
+
+Sometimes, you may have two local working copies of VexFlow (e.g., your own, and someone else's fork). This can happen when someone has submitted a pull request, and you want to compare their output to yours.
+
+You can treat each separate working copy as a "current" build, and run `grunt` and `grunt generate:current` on each.
+
+Now, you will have two `vexflow/build/images/current/` folders.
+
+You can drag these two folders onto the compare tool's page (one to the left side, and the other to the right side). Essentially, you treat one `current/` folder as the "reference" and the other `current/` folder as the "current".
+
+If you find it cumbersome to have two `current/` folders, you could rename one of them to `reference/` and then move it over to the other working copy's `vexflow/build/images/` directory. Then, proceed as if you only had a single working copy, by dragging the `images/` folder onto the compare tool's page.