Add plugin APIs for intercepting URLs

• include/wget/wget.h: Add function declarations
• libwget/plugin.c: Add virtual functions
• src/plugin.c: Add implementation
• src/wget_plugin.h: Likewise
• src/wget.c: Add code that calls the implementation
• src/wget_job.h: Add a variable to store 'accept' action
• tests/Makefile.am: Add libpluginapi.la
• tests/test-plugin-dummy.c: Add a test plugin for testing
• tests/test-plugin.c: Add tests

Plugin API for intercepting URLs has been implemented as per specifications at https://gitlab.com/akash_rawal/wget2/wikis/Stage-2

This merge request is based on branch tmp-akash_rawal-stage1 instead of master.