Add simple img conversion
Test img tag conversion
Add shortcut link syntax
Support inner Options class
Add "autolinks" option

This option allows you to disable the creation of "autolink" style
links.
Remove unneeded raw string
Add "heading_style" option

Allow the user to specify a heading style.
Test ATX and ATX_CLOSED style headings
Add support for "bullets" option
Test nested unordered lists
Test custom bullets
Bump to 0.4.0