Convert B<> to ** instead of __ to allow nesting with I<>
Update tests for changes to B<>
Test explicitly that line-starting asterisks are escaped
v2.001

  - Use double-asterisk instead of double-underscore for bold Markdown.
    This allows bold and italic to be nested without confusing parsers.
    Thanks to Mike Covington for the great pull request (gh-9)!
add command line usage to document SYNOPSIS
Merge two filter examples and comment for Test::Synopsis
v2.002

  - Docs: Add example of command line usage to Synopsis.
    Thanks, Cindy Wang!