Added +_ for space separated properties merge.

[SomMeri]

Pull request for #1756.

Since everybody seems to consider +_ lesser evil, I implemented it. Merging operator are added in order they are written in, so this:

* {
  transform+: first;
  transform+_: spaced;
  transform+: comma;
}

compiles into:

* {
  transform: first spaced, comma;
}

[seven-phases-max]

Looks good, though I'd like to see some decision for #1760/#1764 made before, otherwise it becomes harder and harder to sync (but I'm OK to update #1764 with changes if this PR is merged first).

[pixelass]

So.. any status updates on this?

[lukeapage]

For 1.7.0 I guess

[lukeapage]

merged in @seven-phases-max updated version. Hope you don't mind.

[SomMeri]

@lukeapage Thank you.

[pixelass]

amazing news. too bad it didn't make the 1.6.3 release though.

[lukeapage]

I want to release more often so it won't be long.

[pixelass]

I noticed.. since 1.4.x (something) you guys are really fast with releases. Thumbs up.

[lukeapage]

part of that is actually sticking to semver, so we go up a minor version for new features. Before that we were putting features into every release - so 1.3.1 and 1.3.3 are miles apart.