Add a GNU style formatter to Hadolint.
This formatter enhances Hadolints ability to produce output for
consumption by other programs, notably the integration with Emacs will
become easier. GNU style error messages are
described here: https://www.gnu.org/prep/standards/html_node/Errors.html

This formatter differs in three ways from the default TTY output format:
  - the GNU style has an additional colon after the line number
  - the GNU style has no color output
  - the GNU style includes the name of the program (hadolint) in the
    output, since Hadolint is not an interactive program

Adding a new formatter instead of modifying the existing TTY
formatter avoids breaking backwards compatibility. Since the differences
only matters when consuming Hadolint's output with another program,
configuring Hadolint to use this formatter does not hurt usability
either.

fixes: #766