.mjs should be added as a default extension

[tleunen]

.mjs has been added into Babel so we have to update our list of extensions too.

It would also make sense to reuse the list from babel-core (only in v7): https://github.com/babel/babel/blob/7.0/packages/babel-core/src/index.js#L44

[fatfisz]

In theory this could be imported from babel-core, but does it make sense to make it a dependency?

Or should it just be copy-pasted?

[tleunen]

Hmm yeah.. I guess we can just copy/paste it. It shouldn't change very often anyway :)

[fatfisz]

Feature added in #172.