Make definition lists look like tables

Tables look more clear and isolate inner content from the whole page
context which is quite important for definitions.

This commit:

* adds new xrally_docs_tools.mdutils.Table class for difining tables
  throw python code.
* ports xrally_docs_tools.mdutils.DefinitionsList to inherit from Table
  class.
* removes 'description' parameter from DefinitionsList, since it was not
  used and redundant in case of Table-view
* extends DefinitionsList with term_label and description_label
  parameters which are designed for customization of table header.
* updates CLI and Plugins references.