Add new option to module_supports() to flag modules that support the use of plagiarism plugins

now that multiple plugins support the use of the plagiarism API it would be nice to add a new option to the module_supports() functions so we can check this.

for example - mod/assign/lib.php function assign_supports() should have an item like this:
case FEATURE_PLAGIARISMAPI: return true;
(FEATURE_PLAGIARISMAPI should be defined in moodlelib.php like the other FEATURE defines)

For every module we have added support we should add this to the relevant module_supports function