Reliance on @paulbetts/mime-types is highly inconvenient

[bengotow]

I'd like for electron-compile to support CJSX. I rebased #57 on top of master but found that it still didn't work. The problem is that the require-hook only maps extensions to compilers based on the mime-types database (https://github.com/electron/electron-compile/blob/master/src/require-hook.js#L13), and the mime database does not include an entry for text/cjsx:

If you log the values of ext, it looks like this. Note that text/cjsx is not the only compiler which is broken and not mapping to any file types. text/tsx is also not mapping to anything.

text/less
less
text/stylus
styl
text/jsx
jsx
application/javascript
js
text/cjsx
false         <<<<<<<
text/coffeescript
coffee
text/typescript
ts
text/tsx
false         <<<<<<<
text/cson
cson
text/html
html
text/jade
jade
text/plain
txt
image/svg+xml
svg

Technically, I could submit a PR to https://github.com/jshttp/mime-db to get text/cjsx in there, but they encourage contributors to submit new mimetypes to the governing body according to RFC 6838 section 5. I could do that, but it'd probably take a while, and then we'd have to get a chain of dependencies updated to support it.

I think it would be very preferable to replace all this with a lookup table of extensions to mimetypes. This would also allow for custom mappings - in N1, we use .es6 for ES2016 JavaScript files so that we can track our progress toward the ES2016 standard, and it would be nice for that to be mapped within electron-compile as well.

[bengotow]

Was able to get this working by adding text/cjsx to the overrides already in place on top of mime-types here: https://github.com/electron/electron-compile/blob/master/src/rig-mime-types.js#L8. 👍

If 5/12 mimetypes aren't in the upstream library, maybe it'd still make sense to remove it and just add the other 7 items to that lookup table?