Support left colon bem syntax

[mgreter]

The following

.testing
  :margin
    :bottom 32px
    :foo
      :bar 1px
      :baz 2px
    :baz 3px
  :end 4px

Should result in

.testing {
  margin-bottom: 32px;
  margin-foo-bar: 1px;
  margin-foo-baz: 2px;
  margin-baz: 3px;
  end: 4px; }

To acheive this we definitely need to keep track of the bem partial stack (to unwind it later). Indicators are if line starts with exactly one colon : followed by one identifier.

sass/libsass#1781
sass/node-sass#1300

[mgreter]

This is implemented with f03b821 and is part of v1.1.0