Setup itemize head style
Use metapost to create a item marker

For more details on using MetaPost, read:

- http://wiki.contextgarden.net/MetaPost_in_ConTeXt
- http://www.pragma-ade.com/general/manuals/metafun-s.pdf

For details on symbols, read:

- http://wiki.contextgarden.net/Symbols
Set spacing for itemize

Tweak the spacing around bullet according to your preference.
Example for nested bullet list
Tight itemized list for level 2

By default, lists in ConTeXt are `unpacked`. I prefer to use second level lists
without any additional spacing around the items; hence the `nowhite` option.

This setup is not perfect because of the extra space before the 2nd level
items. This will be fixed in the next commit