Make IO.ANSI.Docs readable on white background

The `default_options` for `IO.ANSI.Docs` uses bright yellow for
documentation headers and titles.

This is very nice and readable when using a terminal with a dark
background. However, when using a terminal with a bright (e.g. white)
background, those parts are unfortunately illegible.

This patch removes the `:bright` part from the `default_options`, to
make the defaults better suited to those of us who uses white terminals.

I have seen the issues and thread about adding an `.iex` file for having
local settings, and I think that's a great idea, but I still think that having
defaults that work for a wider group of developers is a good idea.

Below is a couple of screenshots on the before and after.

![iex_diff_dark_terminal](https://cloud.githubusercontent.com/assets/2308/4952583/60529ff4-6676-11e4-834e-490adff6c46f.png)
![iex_diff_white_background](https://cloud.githubusercontent.com/assets/2308/4952585/631b32d2-6676-11e4-9ff1-f9623680fc1d.png)