Drop NodeJs 6 support & Require NodeJs >= 8.6

[tomap]

This year, at the end of April, nodejs 6 will be 'End of Life'
We are starting to see project dropping support for nodejs 6. and as a consequence, we will need to drop that support also if we want to update our dependencies.

See hexojs/hexo-server#85 as a first example

We should decide if we are ok with doing so, because once we start, we'll have to update in cascade most projects

We should also plan to communicate (blog post) on this

Maybe that could also be a trigger for Hexo v4 #2492

What do you think?

[yoshinorin]

IMHO, current Hexo v4 loadmap has many tasks.
We should narrow down a issue for Hexo v4 & should drop Node.js v6 support.

PS. I think, this issue's priority is higher than the other Hexo v4 loadmap's tasks.

[yoshinorin]

@NoahDragon
Would you please give us advice?

[tomap]

I believe we should proceed with the removal of node 6
When done, we should publish a new major version of each component

I'm just worried about hexo itself, as we were planning lots of new stuff for version 4,
and version 4 might just be caused by the removal of node 6