Get json content type for custom content types

[ggsalas]

I have found a bug using the contentful api. This api returns this content type:

Content-Type: application/vnd.contentful.delivery.v1+json

Currently the content type is vnd instead of json.

I have added this fix, but I think can be improved extending to all the content types this plugin want to support, probably are only 3: json, html and txt. let me know what you think, and thanks for this great plugin 🎉

[NTBBloodbath]

Hey, thank you so much for this patch!

I think can be improved extending to all the content types this plugin want to support, probably are only 3: json, html and txt.

Of course, in fact, I think this little bottleneck was already or can be easily fixed in the development branch (which is rest.nvim's v2, that is close to release) thanks to depending on mimetypes.lua and some Lua pattern matching :)