Allow maximal heading depth do be specified

Adds the -d argument taking a number specifying the maximal heading
depth to be included in the table of contents.

This change pulls the commander Node.js module as a dependency.
Merge pull request #6 from cpetrov/depth

Allow maximal heading depth do be specified
Improve compatibility with some Markdown renderers

Some Markdown flavors (e.g. GitLab Favored Markdown) convert headers
to URLs replacing all characters which are not Latin and numeric
with "-". This change improves the compatibility with them through
adding an "-a" argument option which generates headers for these
Markdown flavors.

This change pulls the commander Node.js module as a dependency.