Specify different bullet chars at different depths

- options.bullet can now be an array of strings instead of a single
string.  Header depth N will use bulletArray[N - 1] as the bullet
string.
- added “npm test” support in package.json
- fixed actual/expected confusion in tests
Merge pull request #20 from TehShrike/different-bullets-at-different-depths

Specify different bullet chars at different depths