Filter syntax for comprehensions

[mlubin]

It would be convenient in many cases to be able to add a filtering condition to comprehensions, something like:
[ f(x) for x in 1:n if somecondition(x)]

where the resulting array only contains those x where somecondition(x) == true. This mirrors python's syntax.

[tautologico]

Haskell, Scala and F# also include syntax for filtering in comprehensions. I believe most languages that have comprehensions support filtering.

[JeffBezanson]

Duplicate of #550.