New: group-exports rule

[robertrossmann]

This adds a new rule, group-exports.

Reports when named exports are not grouped together in a single export declaration or when multiple assignments to CommonJS module.exports or exports object are present in a single file.

Rationale: An export declaration or module.exports assignment can appear anywhere in the code. By requiring a single export declaration all your exports will remain at one place, making it easier to see what exports a module provides.

For extensive discussion, see #721.

[ljharb]

Closing as a duplicate of #721; please reopen that one.

[coveralls]

Coverage increased (+0.06%) to 96.022% when pulling d230d5e on robertrossmann:group-exports into ccd9394 on benmosher:master.