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"
Call the help browser with the correct refresher for afterwards. (From nano commit #4603)

Original nano commit: 4603
http://svn.savannah.gnu.org/viewvc?view=rev&root=nano&revision=4603
Original nano commit message:
	Call the help browser with the correct refresher for afterwards.
Fix formatting
Release 0.0.5
Merge branch 'master' into libtermkey

Conflicts:
	configure.ac
	src/proto.h