Add basic plugin loading mechanism

• configure.ac: Add configure time tests for plugin support backend.
• include/wget/wget.h: Add basic plugin API
• libwget/Makefile.am: Add new file plugin.c to libwget.la
• libwget/plugin.c: Add plugin API functions (new file)
• src/Makefile.am: Add new files dl.c, wget_dl.h, plugin.c, wget_plugin.h to wget2
• src/dl.c: Add abstraction for dynamic loading of object files
• src/wget_dl.h: Likewise
• src/plugin.c: Add implementation for loading plugins
• src/wget_plugin.h: likewise
• src/options.c: Add options --plugin=, --local-plugin=, --plugin-dirs=, --list-plugins
• src/wget.c: Add function calls to initialize and finalize plugin support system
• unit-tests/Makefile.am: Add test-dl, libalpha.la, and libbeta.la
• unit-tests/test-dl.c: Add unit test for src/dl.c
• unit-tests/test-dl-dummy.c: Likewise
• tests/Makefile.am: Add test-plugin, libpluginname.la, and libpluginexit.la
• tests/test-plugin.c: Add tests for plugin support
• tests/test-plugin-dummy.c: Likewise

Plugin loading mechanism has been implemented as per specifications at https://gitlab.com/akash_rawal/wget2/wikis/Stage-0