Using streams only

This is the result of the following issue: https://github.com/wearefractal/vinyl/issues/3

The idea is to deal with streams only at the Vinyl level in order to simplify higher level code.

The API still provide a way to deal with buffers but in a "stream friendly" manner (keeping asynchronous stuffs and back-pressure).

Gulp side effects:
- plugins dealing with streams or with streams and buffers should work.
- plugins dealing with buffers only will break.

Plugins should look like: https://gist.github.com/nfroidure/3631ecc620aec66feb4d
Plugin best practice: https://gist.github.com/nfroidure/19b262e7af577ef84e0c

I know the https://github.com/lmtm/gulp-marked developer. I can use it to write a buffer based plugin migration guide.
Simple version:
https://github.com/nfroidure/gulp-marked/commit/33eec14ae427c782f834016da3b847b23660406c
Recommended version:
https://github.com/nfroidure/gulp-marked/commit/fd34059eda3862bc5bd0108bd495551eb6d01530