only boot inspector on HTML pages (#1137)
only boot inspector on HTML pages (#1137)

Fix #1136.
I chose to whitelist the [HTML mime
type](https://www.iana.org/assignments/media-types/media-types.xhtml#text) only since it is simpler.
Per Godfrey's comment, we could declare [matching in the manifest](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Match_patterns) but it does so by URL or protocol. Because of that I think checking the mime type in the document is simpler and more readable.
emberjs/ember-inspector,sly7-7/ember-inspector,emberjs/ember-inspector,sly7-7/ember-inspector