Add Todo date lint custom rule to lint.swift

[Jeehut]

For a given comment like this:

// TODO: [cg_2016-05-28] find a better way to implement this

The rule could report a warning if the given TODO is between 3-6 months ago and an error if it's more than 6 months ago. That could be achieved with regexes, file reading, but with custom logic using the current date and transforming the date, which shows the strengths of AnyLint very well.