Inline Formatting

I want to make sure I handle all inline formatting. I need to handle
*bold*, /italic/, =code=, ~verbatim~, _underline_, +strikethrough+. 

In addition, I need to make sure I can handle links. We've got simple
links, like this:

- [[http://www.bing.com]]
- [[http://www.google.com]]
- http://www.gmail.com

Note the last one *is not* a link, as the source doesn't include it in
double-brackets and I don't auto-recognize URLs.

I should also handle links with [[http://www.xkcd.com][helpful text]].

Helpful addition from [[https://github.com/punchagan][punchagan]], we now
recognize when the link goes to an image and make the link anchor be the
image, like this:

- [[http://farm7.static.flickr.com/6078/6084185195_552aa270b2.jpg]]

Also, if you make the descriptive text be an image, then it will get formatted
with an image tag, like so:

- [[http://www.xkcd.com][http://imgs.xkcd.com/comics/t_cells.png]]

Helpful addition from [[https://github.com/wallyqs][wallyqs]]:


While "naked" links don't work (like http://www.google.com), angle links
do work. This should look like a link: <http://www.google.com>.

It should be possible to use both kind of links on the same paragraph:

This is an angle link <http://google.com> and this is a bracket link [[https://github.com/bdewey/org-ruby][to a repository]].

This is a bracket link [[https://github.com/bdewey/org-ruby][to a repository]] and this is an angle link <http://google.com>.

This is a bracket link [[https://github.com/bdewey/org-ruby][to a repository]] and this is a bracket link too  [[https://github.com/bdewey/org-ruby][to a repository]].

This is an angle link <http://google.com> and this is an angle link too <http://google.com>.
