Pimp the docs linter (and assorted changes) (#2553)

### Pimp the docs linter (and assorted changes)

 #### `scripts/lint-docs.js`

- Add an optional cache for faster runs
- Add a final report
- Don't return anything from `exec()`
- Cover more files

 #### `scripts/_command.js`

- Look for a "--cache" option

 #### `package.json` scripts

- Added `watch:lint-docs`
- Added `cleanup:lint` to remove the eslint and lint-docs cache files
- Changed `lint:docs` to use the `--cache` option
- Added `test:js` so that we can run the test suite without the linter
- Changed `test` to defer to `test:js`

 #### Actual lint fixes:

- Bad link in a migration guide
- The unicode dashes in the "https://en.wikipedia.org/wiki/Subject–verb–object" are not escaped by marked

### Some more lint-docs pimping

#### `scripts/lint-docs.js`

- some code reorg and cleanup (take a hint from the local coding conventions)
- fix misc bugs
- pass a User-Agent header to the requests
- even nicer reporting

#### `package.json`

- bump the @babel/parser dep to the latest

#### Docs

- tweaks based on lints missed due to previous bugs

### Docs: use the github page for velocity.js, the home page has too many errors.

Co-Authored-By: Isiah Meadows <contact@isiahmeadows.com>

docs/migration-v1x.md

@@ -32,7 +32,7 @@ var Counter = {
     oninit: function(vnode) { vnode.state = 0 },
     view: function(vnode) {
         return m(".counter", [
-            m("button", {onclick: function() { vnode.state-- }}, "-")
+            m("button", {onclick: function() { vnode.state-- }}, "-"),
             vnode.state,
             m("button", {onclick: function() { vnode.state++ }}, "+")
         ])
@@ -47,7 +47,7 @@ var Counter = {
     oninit: function(vnode) { vnode.state.count = 0 },
     view: function(vnode) {
         return m(".counter", [
-            m("button", {onclick: function() { vnode.state.count-- }}, "-")
+            m("button", {onclick: function() { vnode.state.count-- }}, "-"),
             vnode.state.count,
             m("button", {onclick: function() { vnode.state.count++ }}, "+")
         ])