Support @return in mixin

[rentalhost]

Currently we can use a king of hack to return a value from a mixin. For instance:

.sum (@a, @b) { 
    @return: (@a + @b); 
}
.size { 
    .sum(5px, 10px); 
    width: @return; 
    .sum(15px, 15px); 
    height: @return; 
}

But it is ugly.

Suggestion is use the @return variable to literally return a value directly from mixin.

.size {
    width: .sum(5px, 10px);
    height: .sum(15px, 15px);
}

Both should return:

.size {
    width: 15px;
    height: 30px;
}

To avoid breaking changes, both examples should work. @return will keep to set the variable on scope, but too will be used as result of mixin.

[seven-phases-max]

See #538 and less-plugin-functions.

[seven-phases-max]

Closing as duplicate of #538.