[feat] DocumentRegistry: add getProviders() and preferred by weight (#3651)

This is step one toward "open with".

References https://github.com/koreader/koreader/issues/3345

* Fix up some mimetypes
* Add XHTML to supported filetypes
* Add a few image files to MuPDF
	* ".bmp",
	* ".gif",
	* ".hdp",
	* ".j2k",
	* ".jp2",
	* ".jpeg",
	* ".jpg",
	* ".jpx",
	* ".jxr",
	* ".pam",
	* ".pbm",
	* ".pgm",
	* ".png",
	* ".pnm",
	* ".ppm",
	* ".tif",
	* ".tiff",
        * ".wdp",

spec/unit/document_registry_spec.lua

@@ -0,0 +1,23 @@
+describe("document registry module", function()
+    local DocumentRegistry
+
+    setup(function()
+        require("commonrequire")
+        DocumentRegistry = require("document/documentregistry")
+    end)
+
+    it("should get preferred rendering engine", function()
+        assert.is_equal("Cool Reader Engine",
+                        DocumentRegistry:getProvider("bla.epub").provider_name)
+        assert.is_equal("MuPDF",
+                        DocumentRegistry:getProvider("bla.pdf").provider_name)
+    end)
+
+    it("should return all supported rendering engines", function()
+        local providers = DocumentRegistry:getProviders("bla.epub")
+        assert.is_equal("Cool Reader Engine",
+                        providers[1].provider.provider_name)
+        assert.is_equal("MuPDF",
+                        providers[2].provider.provider_name)
+    end)
+end)