add an example of $() to the shell lexer spec
:loop: Swap pre and table tags in tableized output

Previously the HTML Formatter output invalid HTML, as `table` elements are
not allowed inside `pre` elements. This commit reverses the order of the
markup for tableized output such that the `pre` elements sit inside the `table`.

Non-tableized output is unchanged.

Fixes #104
no need to wrap line numbers in a <div> anymore

since <pre> tags are already block
zero out the default 1em margin on <pre> tags
Merge branch 'bugfix.pre-table-swap'