[feature request] Consideration of using async / await

[segayuu]

For feature request

The native promise after node v10 is fast enough.
Since async / await is supported in node v7 or later, it can be used when support of node v6 LTS (maintenance) is turned off.
Using async / await tends to make the code simple, and sequential asynchronous is faster than bluebird on the bluebird side benchmark (in the case of parallel, now bluebird is faster).

[NoahDragon]

Agree. Performance is always the key improvement for Hexo. IMHO, this is not a feature, more like an enhancement. Anyway, thanks for bringing this up.

[yoshinorin]

I agree. Same as other hexo repositories.
IMO we can drop node v6 support after 2019 Apr.