feature: Add compiler directives for deprecation warnings

[nerdgore]

As less frameworks get older and larger there is a need to mark parts of your css as deprecated, making the move to a new version of the framework easier.

Consider the removal or renaming of a selector:

.u-align-left {
  @deprecate(.u-alignment-left)
  text-align: left;
}
.u-alignment-left {
  float: left;
}

The compiler could output something along the lines:

.u-align-left: This selector is deprecated and will be removed, use .u-alignment-left instead

This may be related to #1459 as a specific use case. However, personally I feel that deprecation should be handled on the compiler level.

[seven-phases-max]

Instead of too specific "deprecation message" function, it has to be a generic "whatever you set message" debug/log/warning function. I.e. exactly like in #1459. In fact there's no difference in the "compiler level" between directive and function, both are handled at the same compiler level. Now the only question if this should in the core or in a plugin (yet at least).
In a plugin, one can implement it either as a directive or a function (as soon as current master with "root functions feature" in is released), though personally I'll strongly object against @directive (first Less does not use @ for anything except variables just historically, second there are reasonable arguments against such "pseudo-CSS compatible stuff").

[seven-phases-max]

I don't see any reason to not merge this into #1459.

[nerdgore]

True, closing this in favour of #1459