Make behaviour configurable when jshint is missing

[icemac]

@wosc wrote at 2014-10-22 in https://bitbucket.org/gocept/gocept.jslint/issues/2:

We currently skip tests that need the jshint executable on the system path if that is missing. However, it should be up to the author of the code under test to decide whether jshint tests are indeed dispensable. For example, if there are selenium tests that fail mysteriously under circumstances where Javascript errors might get swallowed, one might want to make sure that the Javascript code looks sane in the first place.