Add first pass at a hydoc commandline app

This adds hydoc as a new script that is a parallel to
pydoc. The main differnce is that it rewrites function
signatures to use Hy syntax instead of Python.

For example, 'pydoc hy.core.language' produces this:

FUNCTIONS
    drop(count, coll)
        Drop `count` elements from `coll` and return the iter

    take(count, what)
        Take `count` elements from `what`, or the whole set if the total
        number of entries in `what` is less than `count`.

where 'hydoc hy.core.language' produces

FUNCTIONS
    (drop count coll)
        Drop `count` elements from `coll` and return the iter

    (take count what)
        Take `count` elements from `what`, or the whole set if the total
        number of entries in `what` is less than `count`.

Functions with optional args get re-written like:

    (foo x y &optional z=10)

Member functions get re-written something like ;:

    (.bar self a b)

This only re-writes function/method signatures, but if hydoc is a
useful idea, it can be extended to change class headings from:

class HyREPL(code.InteractiveConsole) to:

(defclass HyREPL [code.InteractiveConsole] ...

also, note for now that in the REPL (help foo) will call pydoc.
But if you (import hy.hydoc) then (help foo) will call hydoc.