RFC: Documentation System

Ok, so this has ended up evolving into a full doc system. Functions and macros can be documented with
```julia
@doc """
Description here
""" ->
function foo() ...
```
(prettier syntax forthcoming)

Documenting multiple methods of a function will concatenate the doc strings together, in the order the methods were originally defined.

By default `@doc` treats unadorned strings as equivalent to `md""` (i.e. markdown syntax) but you can actually put anything before the arrow to associate it with the function.

Docs can be queried with e.g.
```julia
doc(foo)
@doc foo
@doc @foo # For macros
```
---
*Original:* Here's a first pass at getting Markdown.jl into Base. This will us allow us to display Julia's doc strings in the terminal and rich environments; see the [readme](http://github.com/one-more-minute/Markdown.jl) for some examples.
'flavour' is more fun to write, but it's less commonly used with Markdown specifically, and in languages that don't avoid the word completely, 'flavor' seems pretty unanimous as far as I could see (the exception being Python, which mixes and matches), current Julia codebase included. For a different name, if it fits, though.