#271 Add AsciiDoc source files checker (2nd version)

This is a 2nd attempt, after !278 (closed), now as a Maven plugin:

• It still checks for multiple sentences on a source line, and for indented code blocks. It now also reports empty code blocks.
• It now has a more modular setup. The various things as I/O, parsing to various subsets of lines (normal, source blocks), and doing various checks, are clearly separated. This allows it to be extended with new checks more easily.
• The number of lines of code has barely increased, from 535 to 537, even though we now have more files (and thus more license headers), and test code.

Addresses #271