Add support for code linting via an external program. Adapted from nano commit #4600

Original nano commit: 4600
	http://svn.savannah.gnu.org/viewvc?view=rev&root=nano&revision=4600
Original nano commit message:
2014-02-24 Chris Allegretta <chrisa@asty.org>
        * new linter functionality.  rcfile option "linter"